8 ply is the best for the stock rim, and I am not sure whether any of the tire manufacturers make a 8-ply for the 15" fuchs....I would think by now they would.
If the bus is lowered then the 8-ply is not as crucial as it does not 'roll' as much :)
I don't have the tire number in front of me but I use 15" tires on Ronal rims. The tires were made for sprinter vans and have an 8 or 10 rated ply. They handle well.