That's over 215,000 miles. From what I've understood about these, if they are well maintained, they can do much more than that, but if not, that's quite a lot of miles. That said, finding a mechanic to work on one shouldn't be an issue - they're everywhere in GM vehicles.
For mileage, a 5-6 window van cutaway would get probably 9-10mpg, maybe a little more, depending on how you drive it. A diesel would get a couple mpg more, so if you're planning on a lot of miles, it could add up in fuel costs.
There are a number of threads talking about the 6.0L gas engine, and it's a great engine if going with a smaller gas bus. Just consider the downsides. I did and ended up going with the 6.0L engine, but don't plan on putting lots of miles on either.
I'm sure some of the mechanics on this board will have more and better advice, but this is a start!