You need to pick a system that emphasizes natural length and width stances and minimal stretching. It would also be good to look for a style that emphasizes hands and low kicks, and a good mixture of soft and hard principles. Styles like JKA Shotokan, TKD, Tang Soo Do, Japanese Shito Ryu, Kyokushinkai (this includes Ashiara, Enshin and Seidokaikan) and Japanese Goju Ryu tend to be more rigid and tense and the training includes lots of sweating and free sparring. These are all bad styles for the "joint-impaired". Styles like Uechi Ryu and many Goju styles emphasize body-hardening and this may not be good for the joints either.
Styles which do not require a lot of athleticism or harsh training, but are actually some of the more effective (and original) systems include Higaonna Goju Ryu, Matsumura Seito Shorin, Okinawan Shito Ryu, Motobu Ryu (Bugeikan), Okinawan Shorinji Ryu and Ishimine Ryu, among a few others. Even many of the Okinawan styles which tend to use kata, basics and drills versus a lot of free sparring, like Shobayashi (Seibukan or Sukunaihayashi included) and Meibukan Goju, tend to be more for those without any physical deficit. Kobayashi Shorin and Matsubayshi Ryu can be very hard on the joints if not practiced correctly. Both emphasize lots of hard training, stretching, and Kobayashi uses all the kicks you can imagine (jumping, spinning, multiple, high and low) as well as lots of jiyu kumite (free sparring). It's a great style, but it can be bit on the "external" side of things.
Regardless of what you may think now, if you do a grappling system that could possibly be the worst thing you can do for your joints. Being in orthopedics I have colleagues and patients who wrestled and /or did arts like BJJ and they complain of joint and neck problems all the time. Many are on constant pain killers and have had numerous knee, ankle, and even hip surgeries. One of my friends who is a PA has been involved in NHB stuff for many years now and all the years of rolling has given him the arthritic neck (on X Ray) of a 65 year old man. He's 31. He may need to have his neck vertebra fused and may lose the ability to rotate his head with full range of motion. You need to think about what it is you're doing to your body everytime you roll. Just think about it.
I can't speak for the American Kenpo styles. I do know that good Yang Style Tai Chi is also an awesome and healthy style. Good luck and pick what you see and feel is the best style for you.
