Mark,Prevosts are a damn good bus they were just over engineered in a few places,the biggest fear with a high mileage series 60 of that vintage is what maintenance has been done.
The bull gear and the counter bore are the weak links,the bull gear needs replacing at 700,000 miles or sooner, then if one has excessive blow by from the breather tube it is going to need attention and the injectors are a problem.
Have someone do you a scan on the transmission and engine it's well worth the price.If any work was ever done at DD it will be in their system.I am not trying to scare you but do the home work it saves some big bucks BTDT