Having never owned a "plastic coach" I can't offer any insight on the relative difference between the maintenance of a Prevost and a "plastic coach."
When buying a Prevost I think there is nothing more important than the maintenance history of the coach. Beware of shoddy/deferred maintenance and review the maintenance records of any coach you are seriously considering.
There is nothing more expensive than a "cheap" Prevost!
You have come to the right place if you are in the market for a Prevost, these guys really understand these coaches from a hands on perspective.