There are a lot of good, used, class A rigs at varying price points, and the older units which are still fully driveable and haven't had the snot beat out of them will give you a much better RVing experience than taking a schoolie shell and building it from scratch.
Any older class A that you look at, especially if it is 15, 20, even 25 years old is going to need some work. There will be delayed maintenance unless you find the perfect seller that kept all of it up, but just couldn't use it anymore. Those are pretty rare and go fast. But if you find one that is in decent condition there will be delayed maintenance in some areas that you have to work through. I picked up a much older class A last year. The house was in great condition, but there was a lot of delayed chassis maintenance that cost me a bit, along with the rig needing a new roof. But after spending the money on the chassis and the new roof, I still spent a whole lot less than buying a new rig or buying a used late model rig, and I was able to get out there and camp very quickly after dealing with the chassis repairs.