If you haven't yet signed the final papers on your bus, you could consider something besides a "bus," but built on a bus chassis. I got lucky and found a bookmobile built on a 1995 International 3800 bus chassis. It was relatively expensive, but the benefits of the aluminum body (no rust) and square ceilings more than compensate for the perceived lack of rollover strength compared to an actual school bus. Of course, a good bookmobile isn't easy to find, but you never know what will turn up. Mine was being sold by another member here and I was willing to drive from Florida to Tennessee to pick it up.
And mine also has the much-acclaimed DT466/MT643 combination.
See? You can find perfection!