Just as a question, would you be able to consider an on-chair carry method? It seems to me, though I don't know what your chair looks like, that you could carry off-body but on the chair somewhere. S&W has even hired the first wheelchair-bound competitive shooter, and there is a company that has a line of competition holsters tailored to chairs. It seems that a larger firearm could be considered as well. I'm not in a wheelchair, though, and won't pretend that I know what I'm talking about. It's just a thought to entertain.
I'd go with a .40 over a .45 just because I have the equipment. I've carried everything from .38 to .45, and I haven't felt undergunned with any of them. .357s are an option, but the blast is nuts. I'd pick a .45 over a .357, as the .357 is roughly equal to the .45 on foot-pounds in hot loads in concealable weapons, but the .45 is a bit less harsh.
I'd find a form that you like, pick a caliber, and then pack in something akin to the following: