Re: general bus info wanted
Flat noses are a little harder to work on, generally louder up front, and you can feel a certain amount of engine heat. A custom center console loaded up with insulation will help with #2 and #3. The engine cover (doghouse) also makes it awkward climbing into the driver's seat, and requires an angled stepwell. However, the biggest advantage is interior length. As long as you account for the stepwell angle, your usable floor space is only about 5-6 feet less than the outside length, so on a 40ft bus you could have as much as 35 feet to play with.
If you want an International, look for anything that says Genesis/AmTran/Ward. Power ratings and performance with older buses vary a lot, it goes without saying you should test drive before buying. You won't get the actual 7.3 (T444E in bus applications) in an FE, the V8s are too wide to fit under a 90s doghouse, so they always used the I6 DTs.
If you want a Cummins, you have some choices. Blue Bird and Thomas are preferred. Carpenter offered the Cummins engines, but they had some major quality control issues and it's hit-or-miss with the rest of the bus.
Wayne buses were sometimes built on the International chassis and would have the same DT engines as above. They were also available on the Asia Smith chassis, which should be avoided due to non-standard hard-to-replace parts.
Fuel mileage varies a lot. The one I drive every day at work averages about 8 with mostly rural stop-and-go driving, but that's with electronic controls (2002 model) on a CAT 3126E and a modern MD3060 5-speed lockup transmission. A lot of 90s buses used the AT545 trans - reliable if it's been taken care of, but lower torque ratings and it's a 4-speed with no lockup. Look for something with an MT643 (or MD3060 if you want a newer bus).
As for rear clearance, it depends on the model, tires, and suspension. The buses my employer runs vary more than a foot with rear bumper height depending on the specs. However, one of the tradeoffs with an 84/90pax FE is extreme rear overhang - see Smitty's post in Conversion Discussion about rear decks.