Well, now, Mr. Monstermaker -- if that is your real name....
Still thinking about a bus, eh....
First of all, make sure you get an MT6xx-series tranny, and not the AT5xx-series. (If you find a real beast of a bus, you might get an HT7xx-series.)
There is supposed to be a real Allison guru up there in your area. Power Shift Northwest in Woodinville. Fellow named Harry Mayer. Might ask him.
But I think it depends more on the individual specimen. As Frank said, Allisons are strong, but must be kept cool and clean.
You also want to make sure the modulator cable from the food feed pedal is adjusted correctly.
When you are shopping for a bus, I think all you can do is smell the fluid and drive the thing. Or pay for somebody like Harry to drop the pan and really inspect it properly.
Of course, look for a stick shift!