I drive a 2003 Thomas HDX transit-style rear engine. Inside, it's 96" wide and about 35' long. I'm 74" tall and I have plenty of overhead clearance IN THE MIDDLE
. Naturally, at the edges, it does tend to compress. Raising the roof isn't really a necessity. I have a through-basement for both storage and liquid tanks (fresh & grey). I recently obtained a composting toilet, so no black water tank. Inside the passenger compartment (mine's rated at 84 passenger), you'll find approximately 1400 lbs of excess seating. Once those are removed, you only have the four wheel arches and the two cabin heater units.
I've carpeted/linoleumed(?) the floor and had to have a custom bedframe made to accommodate my full-sized mattress. I just got a 15.5 cu ft fridge that fits nicely in the middle (which is my combination bath/kitchen). The rear is the Master Bedroom, which leaves the front as either a bunkroom or a convertible living room.
Most buses will handle towing a minivan or CRV without even breaking a sweat. I live in Montana and I drove my bus back from Kankakee, IL to Billings at 65 mph, averaging 7.5 mpg.
Engine choices? Personal preference. Mine has a Cat C7, a dog of a motor and one that requires Caterpillar parts exclusively. I'd prefer either a Cummins or a Detroit. The Allison tranny that is in mine is a beauty, shifting quite well.
When I got my bus, all 6 tires on the ground were almost new. Truck tires are VERY expensive, so this, in the end, came out well. The batteries may need replacement sometime in the future, but not right now.
Where I finally got parked has shore power, so the inside can run comfortably on a 30a circuit, although I'd recommend a 50a system. My cook-top is an induction cooker. I've already survived a Montana winter in this beastie, but I would like to be just a skosh warmer when I get out of bed. I'm looking at a Russian hydronic heater (about $900) that'll run off the onboard diesel tank at 0.7 gph. But, who needs heat in the summer?
I got my bus through Midwest Transit
for $8K. They specialize in school buses and activity buses. They're there to answer any questions, too.