If you have back problems, gunsmithing might not be the job for you, other factors aside. Most gunsmithing work is done on the feet, bending over a lathe or milling machine or drill press. Bench work involves bending over the work, sometimes for long periods of time as you tediously file and fit a replacement part. I did it, but now have lower back problems and probably could not do it today without constant pain.
Before spending money and time on a gunsmith school, I would try to locate a local gunsmith who will allow you to sit in and observe his work for a day or so. Tell him what you want to do and perhaps he will let you actually do some work if he feels he can trust you. I don't think he will be worried about possible competition; most smiths fully recognize that there is a lot more work than there are good smiths and are more than willing to share their knowledge.