I don't have a WV bus, but I have a Virginia Bus, and the climate is the same. I agree, they aren't geared for high way driving, and will need some adjustment. Issue can be fixed for $1k-2k by replacing gear ratio inside the differential, so plan for that. Also need to consider if you are running an Allinson 545 or something better. Highway gear ratios with a 545 will heat them up more, and would require an upgraded transmission cooler as well.
As far as rust goes. Mine was mostly rust free for a 28 year old bus. It was in really good shape, so it depends, you'll always need to inspect them. If you are buying blindly from an online auction, I wouldn't buy unless there's good and plenty of under carriage photos. You can always post them here and we can tell you if it's good or not. If there's only a small section of rust it can always be repaired by cutting it out and welding in new sheet metal, but it's a matter of how much of that are you willing to do.
Also remember, all buses have a little rust. And rust doesn't stop growing unless you cut it out. But if the undercarriage looks mostly good, it's probably okay to buy it. All floors under the rubber have a little rust. All kids spill their juice on them.