You have surface rust everywhere on the underside, the result of being exposed to salt zone weather for thirty years. However, your real problems are going to come in areas wherever your roof, light openings and windows have allowed water inside the bus which soaks the plywood on the floor and eventually corrodes through the steel floor from the inside out (like in this bus that some idiot on this site actually bought
). Your photos don't really show any of this potential damage and really they can't (your third photo sort of shows what seems to be the typical corrosion that occurs around the driver's seat), so the only way you're really going to be able to tell is by poking around the underside with a screwdriver like Jolly Roger suggested. Or you will definitely know how bad the problems are once you remove the seats and the plywood flooring. This sort of damage is usually concentrated around the wheel wells, the back of the bus, the driver's seat and the step well - all the places where water will tend to flow and accumulate due to gravity.