Depends on how you look at it. Fundamentally, I'm looking at the overall condition of the bus (particularly regarding rust damage), the type and condition of the engine and transmission, and the condition of the tires. The amount and type of rust damage play a big part in the suitability of a bus. Different types of engines and transmissions are better suited to different things. Mechanical engines are easier to work on than electronically controlled ones. Tires for a bus are expensive.
More superficially, I'm looking for a bus in the 30-35 foot range. I prefer a Thomas at this point, but I'm somewhat flexible if the right bus comes along. I'm going back and forth between favoring a conventional (dog nose) bus and a flat nose front engine bus. I'd rather have the emergency exit in the rear. Beyond that, I'm relatively flexible still!