Re: Total newbie, some pointers?
Don't limit your search to just "school buses." I found a 1995 bookmobile through the classified ads on this site and I'm very happy with the platform. It's based on an International 3800 school bus chassis, but the body is all aluminum with over 7-1/2 feet of interior headroom across the flat ceiling. No need to raise the roof or build crazy-shaped cabinets to fit the rounded ceilings of a typical school bus.
I'll admit these are a lot harder to find than a used school bus, but sometimes they pop up. You just need to be ready to grab it when you find it. I took a Greyhound from Florida to Tennessee to get mine and found the interior to be a mess, but the basic platform is a great starting point. Mine has a DT466 engine and Alison MT643 transmission and gets an actual 10.3 MPG at around 55 MPH.
So, be patient and look around, but be ready to jump on a good deal.