Flat nose vs conventional.. that's always a big debate for people. Conventionals to most are considered easier to work on in the event of engine troubles, or even just general maintenance. Rear engine transit buses are great for their quiet ride, but they're definitely a bit tougher to service depending on which manufacturers bus you purchase. For a lot of long distance, long hour travel I would suggest a rear engine bus made by Blue Bird or Thomas, as the AmTran/IC rear engine buses tend to overheat on long trips due to their design. With the Thomas or Blue Bird you'll end up with either a 5.9 Cummins (most likely) or a CAT diesel (models vary). The plus side to a Blue Bird or Thomas over the AmTran/IC rear engines is also serviceability, they have access to the rear of the engine through a back service door, whereas the AmTran/IC buses (pre-200
have a radiator mounted in the rear, which allows only for access to the engine from the sides of the bus.
As far as frontal impacts go, you'd be surprised how sturdy the front end of a transit bus is. Certainly you don't have the engine and hood in front of you, but in the same sense that's also a good thing. I've heard of accidents where the engine protruded into the cab instead of falling below the bus as designed. After having run so many hours.. having a hot engine shoved into your legs wouldn't feel too good I can't imagine. That's obviously a rare situation, but it's happened. You need to think about the types of buses you see on the road doing long distance trips, or even local. Mass transit and long distance charter buses are transits for the sake of offering visibility to the driver.
Finally, there are plenty of places you'll find buses with air conditioning, depending on how far you want to travel to get the bus. Below are a couple of places that sell used buses:
http://www.midwesttransit.com/Used/Used_Default.asp - Kankakee, IL - South Holland, IL - Whitestown, IN - Eaton Rapids, MI - St. Louis Area
http://www.riversbus.com/used-buses-for-sale.aspx - North Florida
http://www.a-zbusused.com/used_buses.html - Colton, CA
http://www.bargainbusnews.com/search.php - Springfield, PA
http://www.floridachurchbus.com/ - Hudson, FL
http://www.nationalbus.com/ - Nationwide Services
www.422sales.com - Listings of upcoming auctions, next one is in New Haven, CT on Saturday July 16 - I'll be there
(Just to clarify, Blue Bird, Thomas, and AmTran/IC are considered the big 3 which is why I only mentioned those. There are other rear engines floating around, but they're not at all common, at least not anymore)