Take it for a test drive on the highway and see what it does, like max. speed and what rpm it takes to do 60 mph. Have someone follow you looking for smoke. Check the the tires, they cost about $500 each, and read date codes if you can (ten years would be absolute max age). If 330k is engine miles, it's probably time for a rebuild (& probably tranny too). $$$.
Of course $5500 is too much for a worn out bus, unless there is something in the conversion that's worth it. That doesn't appear to be the case here.
$2500 bucks can buy a really good school bus that has just come out of service.