Unless some of your crew are accomplished mechanics I would stay away from any of the older buses. It isn't that they are harder to work on than newer buses but finding the parts and pieces to make things work is getting pretty difficult for some of the makes.
IHC still supports most of what they have made over the last 100 years. You would be amazed at what is still available at an IC Bus or Navistar truck dealer. Or from your local NAPA, CARQUEST, or O'REILLYS. But I have absolutely no clue where one would start to look for parts for a Studebaker, White Super Power, or REO. It is hard enough to find parts for a Dodge, Chevy/GMC, or Ford.
You will also find that not only do the older buses have brakes that are quite a bit smaller than newer equipment but the engines are also much smaller and have much slower speed gearing. When the top speed is geared down to 47 MPH you don't need nearly as much HP and torque as you would need if the top speed is geared to speeds in excess of 70 MPH.
There are some buses out there that might be what would work for you. Recently someone posted a '60's vintage Crown 10-wheeler for sale that had Cummins power. The price was very attractive. And I know parts and pieces for a Crown are not that difficult to find if you know who and how to ask for parts for one.
Good luck and happy trails to you!