They came in two flavors. The first wasn't so very good and the second one was pretty good.
The basic engine was great. It was the operating systems that were trouble prone. By the time the new clean air regs came along M-B decided to go another route than to try and clean the 906 up.
In a used school bus, by the time it will hit the secondary market all of the bus will have been worked out and it should be a pretty reliable and economical engine to run.
However, as it was already pointed out, parts and pieces won't be cheap. And finding them will be a challenge.