Tex,
First thing I would tell you is to find another gunsmith! AR's are relatively simple to keep running and I'm surprised you are having issues with it. Have you contacted Bushmaster? They will stand behind their product and like many, I've never had a problem with any of my Bushmasters....but I digress.
Any of the major AR manufacturers produce a very good AR
Armalite
Bushmaster
Colt
DPMS
Rock River
Sabre Defence
Smith & Wesson (as funny as that sounds!)
Stag Arms
CMMG
Plus a few others I'm probably forgetting. I've had experience with all of them and they all worked well. My favorites right now are Rock River & Sabre Defence, but I would use any of them.
One manufacturer that I'm very happy with is Patriot Ordnance Factory (POF). POF makes a piston-driven upper. Think of it like an AR with a FAL style gas piston. The AR pictured in my avatar is a POF M4. If you really want a do all setup, the POF is very hard to beat. They are more expensive, but they perform and talk about easy to clean. I once went 1,200 rounds without cleaing just to see what would happen. When it came time to clean the rifle. I took a rag and wiped off the bolt and carrier and it was clean! No scrubbing, scraping, gouging, or cussing. It would have easily gone another 5,000 rounds. I'm really sold on the piston driven systems. One other thing about the POF's. They are quite accurate, but they are not sniper rifles. You will get outstanding accuracy (1 MOA or better in some instances) for a combat rifle, but if you're looking for precision rifle setup, you'd be better served with a standard AR with a match grade barrel.
As for your use of a cane, it really depends on what courses you are interested in taking and how much you rely on the cane for mobility. Give me a call at your convenience and we can privately discuss your situation. USSA will make every effort to accomodate you, but I want you to know what our courses require so you'll have all the information you need to make an informed decision as to whether it would be worth the effort to travel to our facility.
Erik
703-798-2226