First of all, I would say buy neither. Much better to buy a newer fresh bus from a school district.
But of course, newer buses can cost several thousand. What are these guys asking? If it's a couple hundred bucks, then it might be worth a try.
Three things: Condition, condition and condition. Engine and tranny and tires and suspension and so forth. Then there is all that old conversion stuff in the International. Does it work? Will it continue to work? In short, I'm leery of somebody else's project that he gave up on.
Maybe I'm all wet. I'd just hate to see you spend any "real" money just to inherit somebody else's problems.
Also, what are you going to do with a bus? The intended use can make a lot of difference in what you can put up with and what you should not get into.