Your question is a little too open ended to have an definitive answer.
The premier choice would be a Type 'D' rear engine bus with big HP, highway gears, pass through under the floor luggage compartments, low miles, and no rust.
On average, the least expensive choice will be a Type 'C' bus with a power package that lends itself more to to/from routes rather than a trip.
You need to first determine what your have to haves are, what your what it would be nice to haves are, and what your really do not want to have.
Once you have determined those parameters it will be much easier to make an educated choice.
Good luck and happy trails.