I doubt a forty footer with 300 hp is going to be any less economical than another.
I'll give you the tag axles raising the cost, but coaches are DESIGNED for cross country travel, a school bus is not.
THe differences between real coaches and school buses are immediately crystal clear as soon as you are the one driving the school bus a thousand miles.
That detroit engine is described as awesome or legendary if its in a Crown. Two stroke diesels are pretty awesome. They are pretty common, and will likely be cheaper than a new cat c7 as in the 2008 bluebird. How economical would rebuilding a dt530 or any other modern schoolie motor in an RE be?
I'm all about school buses, but if spending four or five grand on just the bus, I think it starts to make more sense to fly coach, so to speak.