You will get plenty of good advise on comfortable bikes and setting one up. As a life long sufferer of a "bad back" my advice has to do with how to deal with it. There may well be some limitation in your case, but in general, strengthening core muscles is what supports the body, and unloads weight bearing on discs. This is also true for riding a bike. In any case, after I began daily core exercises, my back problems diminished considerably. I will always have a bad back and parachute landings are beyond me, but I am mostly pain free.