I see there are buses for flat land and mountains, are there any middle of the road, engine and trans combinations, that can give a little more power without eating up so much fuel?
I haven't really seen much more than about 2mpg between any of the combos out there. If I was looking for a middle ground, I would sway toward the better drive trains that will take you anywhere. Find one with highway gears and overdrive trans. That's probably more important than motor choice. A bus that can do 85mph gets decent mileage at 60mph.
The big motors will actually give just as good fuel economy as the smaller ones on the road, sometimes even better since they don't have to work as hard to get up to and maintain highway speed. It's been said that coach-style buses (think Greyhound style) often get around 10 MPG if driven responsibly and many have big motors.