I’m brand new here, came to look around for inspiration/ideas for a short bus conversion. I have experience with the GM 6.0 gas engine. It is a rock solid performer, and is sought after in classic pickup trucks (my other hobby) as well as other old cars as a modern swap. It has a 300K mile duty cycle, so I wouldn’t worry about buying one with 143K miles.
I live in NJ as well. Can’t comment on how the salt affects bus bodies, however you should inspect the fuel and brake lines on the vehicle. The brine solution they’re now using here really gets everywhere and fuel/brake lines are a soft target (no pun intended). The good news is if you are mechanically inclined they aren’t that hard to replace.
Disk brake calipers are also a potential rust issue, and salt has a habit of drying out ball joint boots and other rubber parts, but I’d expect a 15 year old truck to need all those things freshened up anyway.
Looks like the exhaust system has been repaired, so the district/company that owned that bus kept up with maintenance, which is a good sign.
Frame rot is unlikely unless the bus was really abused and not cared for.