How Old to Buy?

I would go the opposite direction. Get a late 90's Safari or Beaver Class A Diesel. 1996 saw the introduction of slides, and these can still be had for well under $50k. The more TLC it needs, the cheaper it should be. Mine, for instance, was only $5200 in the end, but it needed a LOT of TLC (new roof, new tires, new fridge, new convection combo and a total remodel of the kitchen and bathroom) and is a 1995 with no slides. Once you have the coach, get the roof rebuilt if needed, otherwise get the rubber/fiberglass/whatever replaced with a spray-on roof. They come with 10-20 year warranties and are allegedly maintenance-free. Replace the tires with new Firestone FS580 PLUS radials (good, yet budget-minded) and enjoy the road.

You will be much happier with a 36+ foot diesel coach in the long run.