Finding an Engine:
1) Dog-noses are cheapest to work on, then flat-noses, then pushers. If the engine is in the rear it's harder to work on and costs more.
2) Everybody loves the Cummins engine, super reliable, and it's in a lot of big rig trucks too so it's easy to find parts. Everything I've read says if you can find one in great condition it'll last forever (if you do your maintenance religiously)
Caterpillar engines are equally reliable, but parts tend to be a little more expensive, and as they're usually used in construction machinery (think dump trucks) a lot of the garages that work on them can
Internationals and Detroits are well-respected too, I jut don't know as much about them. They're cheaper options than Cummins when bought new and I don't think the Internationals have quite as much power (I've seen a lot of Internationals in mid-size buses, where I'm looking)
I'd stay away from a Mercedes engine. Not sure why german motors are ending up in American school buses, but german imports are usually more expensive to maintain.
3) GET THE bus INSPECTED before you buy. It'll cost $100 or so but can save you thousands later. If you're buying locally, visually inspect it yourself before paying the inspector. (
I've only really heard about the Allison (people love it). Just google the transmissions of the buses you're considering to check for duds, and of course get it inspected before purchase.
The only way to know is to test-drive. Some districts put speed caps on their buses, some don't. Some skoolie builders have been able to remove the caps, some haven't. If you're buying directly from a school district they may know how to remove it.
1) No one really has much to say about the body manufacterers that I've seen...the key is to find something you like and take care of it. Treat all your rust properly with Corroseal or Rustoleum when you get your bus, and keep up on spots that appear later.
2) If you're going to be off-roading you won't want to mount too much stuff under the bus, at least not that hangs down below the natural bottom clearance. The lower that stuff gets, the smoother you'll need your roads to be. You don't need a super high clearance for most national parks, though, they're usually designed with RVs in mind.
Year rounder for any climate
1) You're going to want to consider your heating and cooling plan EARLY. Heating and cooling are the biggest power draw in a bus, if you're going to be boondocking in unfavorable climates it takes planning. Good insulation, high efficiency equipment, and a large battery bank - you don't want to go out and run the generator in the middle of the night because it's too uncomfortable to sleep. You will need a generator. You can run an air conditioner off solar alone but you have to design your ENTIRE RIG around it and it will cost thousands of dollars more than just running a generator for a bit each day to recharge. And remember that domestic air conditioners are more efficient than RV units. Mini split systems are best, but even a window unit is going to be better than most RV rooftop conditoners. Look at the R value of your insulation, and make sure you consider moisture control. You can bet that tin can you're building inside of is going to sweat. That means mold and it can even ruin your insulation if you get one that doesn't like water (read: fiberglass). I'm going with spray foam in mine; it's more expensive but it has the highest R-value and it's a moisture barrier in and of itself.
If you'll be connected to shore power most of the time those considerations get a lot smaller. (Do watch out for mold though that never stops being a consideration)
2) PSA, if you get a propane heater get a direct vent
propane heater so you don't get CO poisoning. And get a CO detector regardless. I've seen a lot of rigs with no CO or fire detectors. It's probably more important in an RV than a stick-built house. Get them battery operated, and independent from your house batteries. Test them. Maintain them. Don't die.