New vs. old, long vs. short, its up to you and what you want. A newer bus will have more electronic "Doo-Dads" that can fail, but they should also have fewer miles and be generally nicer and can be more comfortable. Older buses are going to be simpler, less electronic stuff, more manual controls, less electrics on the engine and more basic. However they will have more miles(often) and less creature comforts.
As far as short vs. long, your going to get more engine (size,power) in a bigger bus, a short bus will have a smaller engine to move less weight. Be sure that the bus you pick, if you go short, will have the power to haul the extra weight of a trailer, many will not.
For every rule there is an exception, you may find a short bus with a big powerful engine, or an old bus with low miles, a new bus with basic creature comforts etc.
Also consider where you will be taking it, the length and and turning radius of a long bus vs. a shorter one with 24 ft trailer. Camp grounds may have a total limit on the length also. 30ft of bus and 24ft of trailer, 6ft of tongue and you are at 60 feet! however a long but is 40 feet.
Also think about if you want front engine or rear, flat front or conventional (dog nose). A rear engine can not be converted to a flat bed to haul a car, front engine can. Many rear engine buses are long (40ft-ish) and powerful.
How much do you plan to camp in it vs. using it as a tow truck? Are you in Texas where it is all flat (or Kansas) or are you going over the Rockies with it? I can tell you a 30 ft with a 24 ft trailer hauling a car over the Rockies with a 5.9L Diesel or an old gas engine would be a VERY slow trip.
My personnel opinion would be to find the bus with the biggest most powerful engine you can get in a front engine bus, then go from there.