I never did real estate photography. But when I used to photograph crime scenes, I used as wide a lens as was needed. That could be an 18mm, 24mm, 28mm, or 50mm. I don't think there were 18mm to above then, at least not usable for what I was doing. What I found more important was viewing distance, so a prosecutor could instruct a jury at what distance to hold a print for no distortion.
I think that isn't perhaps so important in real estate photography. Just don't be too obvious that you are showing apparent roominess where it isn't, or at least warn clients of the effect of any wide angle lens you use. They will after all wish to see for themselves. People appreciate agents that are up front with them.
Back then, I used film only (still do mostly). Now, as mentioned, more people want to use digital and it has many advantages. Lenses are better now, and many zooms are quite usable. In fact, I have a film kit with an 18mm to 28mm, 28mm to 70mm, and 75mm to 150mm. Those lenses are surprisingly good and get most of the shots I want.
Do get a tripod. Real estate photography isn't racing photography. You can take the time to get the focal length, depth of field, and any distortion out of the way as much as possible.
Good luck on your retirement and 2nd job venture.