I still own 1972 Prevost conversion even with family "shrinking" we moved to cabover camper.
Did not drive "the bus" for 3 years, but when the potential buyer wanted to take it for test drive, some tire pumping and new batteries were all I needed.
The only problem was some cleaning of electrical connectors here and there.
Old Detroit after sitting for 3 years took 2 seconds to fire up.
I think it is not the age that matter in this choice, but quality of coach you are considering.
I am asking 20 grands for fully usable Prevost ;)