Your description of a "decent GMC 1997 bus for $1800" isn't quite enough to go by, much like you'd look at any used car.
This is my first diesel bus after a number of short gas buses and I by far prefer diesel. This bus, I mean van, is automatic rather than stick as in the other gas buses I've had. Besides, with a diesel people don't keep trying to steal your gas. A 40 gallon gas tank says "come and get it."