You can LOOK at any bus anywhere but be very careful buying in any state that salts it's roads during winter. If you look at medium duty chassis buses it's easy to get under them. Throw down a tarp and crawl under with a good flashlight, camera and a cup of coffee and make a very thorough inspection.
I bought a bus in NJ that is very solid underneath but the interior floor was rusty due to the salty schmutz tracked in by the kiddies. I probably spent half and hour or 45 minutes under my bus before I bought it, just poking around. I also had my neighbor who is a heavy equipment mechanic for the state spend some time under it too just in case I missed anything.
What neither of us could see was the rust that was working it's way down from the inside because that salty schmutz soaked into the 3/4" rubber membrane covered plywood floor where it never dried - a BAD THING. Let the buyer beware. I stayed away from anything that was low enough that I couldn't easily crawl under it without jacking it up - for several reasons.
I might also add that floors rusting from the top down isn't a problem exclusive to states like NJ. If I recall correctly someone here mentioned they bought a bus in Texas that also had rusty interior floors due to a membrane covered plywood floor.