OK, you are asking a number of questions which both stand alone and are connected to your ultimate goal. I'll try to answer them individually and see if I can make it all tie together in the end. In full disclosure, I have not trained with the Monk Spade.
Well all I can say is the vendor told me that specific spade I linked to, they said is made of carbon steel (but I'm not sure if that's just them making that up or what really? I don't know much about steels other than hearing stainless is bad for longer weapons because of those longer lengths causing breakage on contact. Is there a rough guide on how long and wide a weapon should be before it shouldn't be stainless anymore? Could you enlighten me on the matter? Since you seem to know quite a lot, by what measure can one say "x" weapon is suitable enough for combat when it comes to facing any other weapon if there is a such a thing? Do you know any reputable makers who would build me (or offer) a real combat monks spade and training version like the wood one or maybe polypropelene and how much should I expect to pay? If not would if be feasible to just try and make a wooden training one?
I've never made a blade but I have rebuilt a number of hilts and scabbards for swords. I understand some fundamentals of the steel work, but I do not have those skills nor the workshop to actually make a blade from steel. I have always made a point to practice with real weaponry, so quality and appropriate dimensions and balance and materials, etc. has always been important to me.
There are a number of different "stainless" steels, each with its own alloy that resists corrosion. I believe chromium and/or nickel can be an additive in the alloy (not a surface plating), but there is some variety in different stainless steels. They are not all the same and their level of resistance to corrosion varies from one alloy to another. The additives that give the alloy its corrosion resistance also tends to break up the grain pattern of the steel. This results in potential weak points within the alloy. For a small to medium sized knife, this is often not a problem as the knife is not meant to withstand very high torque and tension and shearing forces. If you use the knife as a crowbar then you might break it, but it isn't meant to be used as a crowbar. General cutting and whittling and use as a general purpose tool is just fine. It is also critical for surgical scalpels because the introduction of rust is very dangerous to the sanitary environment of surgery. However, for longer blades like swords, this can become a problem. The very length of a sword blade means that it is likely to undergo higher stresses on toque and tension and shearing and whatnot, and the alloy can break under those conditions. This is why stainless steel is often discouraged in the making of sword blades. However, as I said not all stainless steels are the same, and some can be used to make a quality sword blade. A skilled maker who is experienced with the steel can do it, but it is often not recommended for a novice.
So I can't give you a definitive answer to what is the maximum size of blade that can be done with stainless steel. It just depends on what the tool is meant to be used for and under what conditions, etc.
Generally high carbon steels are recommended for good quality sword blades and axe blades, which the monk spade would be similar to (the axe blade). There are a lot of different alloys with different properties, that all can make for a very good quality large blade. I am no expert on that so I won't go into the details. However, these do rust when wet or if you get sweat on them, so when you are done practicing, you wipe them down and then wipe on a thin coat of oil, something like wd-40 in the US, to inhibit the rust.
If you really want to spar with a Monk Spade, i suggest you build one out of a good pole and cut the blades out of a light plywood, perhaps 1/4 inch thick, and thoroughly round out the corners so there is nothing sharp on them. Make sure you have figured out a strong way to attach the blades to the pole, and then cover them solidly with some kind of foam padding and heavy duct tape. Make sure to inspect them regularly to make sure a corner isn't poking through the foam and can become hazardous. And then invest in some body protection equipment, like protective gloves and head/face protection. And still, without a good instructor guiding you, I don't recommend this. I think there are still a lot of opportunities to get really hurt doing this.
regarding a "real" weapon, there is a range of what that might mean, and it might mean something different to different people. Quality construction is important so the whole thing does not come apart. For a bladed weapon, the blade ought to be made of a quality steel, but that does not need to mean a super high-end expensive steel. But it needs to be properly shaped and heat-treated and balanced, etc. to make for a good durable, useable weapon. There are quality steels available for a lower budget. The blade needs to be an appropriate size to be realistic for the weapon, and also appropriate for you, the person using it. What might be right for one person can be far too heavy or too light for another. So there is a personal element in what is right and not right.
Again, look at the link for the inexpensive Monk Spade that you posted, the 47 Euro item. Look at the closeup of the blade. It is thick, but the whole item only weighs a kilo or two. As I said, if that was a real blade it would be solid steel, and at that size and thickness, it would be outrageously heavy. and then there is the crescent blade at the other end, adding additional weight. The blade on that item is made from some kind of sheet metal welded or soldered together into a hollow shape. It is light. If you tried to sharpen it, you would open up the seam between sheets of metal and see the hollow space inside. If you struck something with it, you would dent it and perhaps crack the seams and break it off the staff.
Now look at the more expensive item, priced around 159 Euro. Look at the closeup of the blade. It is a flat solid steel blade. It isn't built up into a thick piece. This is a real blade, although I cannot speak to the type of steel nor the quality of it. But at least it is a real steel blade, and the item weighs a lot more than the other one. That is because steel is heavy, and a real blade like that can weigh a lot. And I don't know how well it is attached to the staff. I always worry about heavy blades like that coming loose or flying off during use. Someone gets hurt.
Could someone make you a real blade from quality steel? Perhaps. I bought a couple knives from a fellow in Nepal, he had a shop on Etsy.com which is closed now, but he has a website where he and some other people sell a bunch of kukhri knives of various sizes and designs. He uses a good kind of steel for big blades, 5160 spring steel, which is a tough steel used in leaf springs for truck suspensions. I have a few swords made of this steel, it is good stuff and not terribly expensive. Anyway, I know that on Etsy he was taking custom orders for knife designs, so perhaps he might be willing to make something for you if you give him a design and have a good idea of how to attach it to a good pole when it arrives. You would need to put it all together, if he/his group is willing and able to make the blades for you. Check out their stuff at everestblade.com. and their prices are pretty reasonable for big knives, I think it is the Nepal economy compared to the US economy. Of course in the current Covid-19 pandemic I have no idea if he is currently working.
Regarding your desire to spar, you just cannot do this with a real weapon. Assume the 159 Euro item is a "real" (enough) item and weighs in at around 15 pounds. That's a pretty heavy item to be thrusting and spinning around. The weight of it, and the momentum you would build, would make it very hazardous and difficult to control it. I don't think you could hit your partner lightly enough to not injure him, even with some protective clothing. If the blade is sharp, that's just hazardous, and even if it is blunt, if you hit someone you could smash fingers and crack skulls.
But if you learn the method from a good instructor, you can practice with the real one for your technique and forms, and then spar with a safe, light, padded one. But understand that the light item will handle very differently from the real one, so this kind of training is only an approximation, which is the best we can do. what you can do in sparring with a light item, you simply cannot do with a real item that weighs 8 or 10 times as much. People get used to using a light weapon and they think they could actually fight with that same speed and control and whatnot. Then they pick up a real weapon and they are in for a big surprise, they simply cannot move as well with a real weapon because they are not used to it and have not developed the proper strength to use it skillfully.
BTW I never understood this whole mentality about having to learn one weapon before graduating to another when it comes to weapon training. Can you elaborate about that a bit? Couldn't "x" movement taken from one weapon possibly be adapted to only the weapon you want to learn, like the monks spade and not staff for example?
strictly speaking, I don't think you need to. You could simply be taught the one weapon you want. And it isn't about graduating from one to another so much as recognizing that some weapons are more difficult to work with so starting with something simpler is a better progression. So usually a good instructor is going to lay down the foundation first and make sure you have a basis upon which to build the later skills, i.e. the Monk Spade.
Also I would like to find a good trainer, but seem to find a lot of suspect schools in my area in terms of how they train and lack of safety precautions I've trained at.
Where do you live, and do you have some websites of schools in your area? Maybe someone here can offer a suggestion of they know any of the schools around you, or at least we can look at the website and give our impressions.
How could a beginner decipher a good trainer and on top of that a lot of chinese martial arts schools I've seen in videos seem to train with those flimsy wushu weapons. Shouldn't they be sparring with real combat weapons if they are at that level of training or if not the wooden training ones you mention?
The fact that you recognize those flimsy wushu weapons for what they are, is promising. They are not real weapons, they would not stand up to contact training, they are simply stage props. The 47 Euro Monk Spade that you linked to is also a flimsy wushu stage prop. The first time you picked it up, you would realize what you had in your hand and that you wasted your money.
I would say that most Chinese martial arts schools, even good ones, do not spar with weapons. There are just too many safety issues to be able to do that and investment in protective gear can become expensive. However, they ought to do some kind of contact training drills with partners, using reasonably safe weapons.
It's kind of the reason I want to consider videos instead and find a training partner but am not closed to the idea of an actual good school. Again I'm a beginner at all this, but I ask questions and cross reference too on things that might seem dubious and without purpose like anyone else should.
Again, as a beginner, you really do need to get a good teacher. The Monk Spade is not terribly common I'm afraid, so even if you found a good school there is no guarantee that they teach the Spade. I think that also makes it even more difficult to do on your own. But you need a teacher to guide you and give you the appropriate foundation first, before you even start with the Spade. If you had a number of years in as a kung fu guy and you had some solid skills built up already I might say experimenting with the Monk Spade could be an interesting activity. As a beginner, I suspect you just don't have the frame of reference to understand what I mean by that, but it's true, it's genuine honest advice. I think trying to figure out the Monk Spade, and then start sparring with it with your friends, is a bad idea and could be quite dangerous. Really, no kidding.
