I would not totally rule out a used motorhome. Motorhomes like trailers use all the same components so virtually any piece is readily available. The vulnerable part is the engine. We bought one that was four years old with 20,000 miles on it. It was like new and 9 years later the engine still runs great and to this point we have not replaced anything except the front TV.
If I were doing it over I would buy a used GMC Motorhome, one of those original all fiberglass units. I think they're beautiful though smaller than our Bounder. The outsides of the units seem to last foreever and there is a real solid supporting club.
Over the years we've looked at all kinds of Motorhomes, particularly some of the classics. I'm glad I'm not rich or my little 60x100 yard in NH would be filled with Rigs.
It certainly would be fun to do a conversion, but I think that the cost would be probably as much as buying a used motorhome. Most motorhome owners put very few miles on them and particularly now it's possible to find a good deal.
The editor of the Gypsy Journal has a multi part article on a conversion he does. Both of these sites deal with convential buses but I bet there are sites on school buses as well.
This one contains some school bus info.
This one contains lots of images of converted school buses.
Good luck in your hunt.