Engine

Engine compartment should be clean and look stock. Too many "custom modifications" could be hard to undo and their reliability is questionable.

Suspension

The suspension should be "tight", ie there should be no or little gap in the joints. When driving the gap can be heard as clunking sounds when going over bumps, or felt as a deadband on the steering. When parked the steering wheel should show resistance when turned in both directions. A no resistance zone of more than 2cm is not good.

When checking under:

When jacked up do the wheels turn freely? Check brake discs (by unbolting wheel or peeking and feeling behind the tire): Clean, no deep rust? No deep groves?

When jacked up do the wheels turn freely? Check brake discs (by unbolting wheel or peeking and feeling behind the tire): Clean, no deep rust? No deep groves?

Uneven tire wear indicates some kind of suspension misalignment. Pulling to one side when driving is also a suspension or brake problem.

Underside

What condition is the underside of the bus in? Severe rust? Or still coated with intact black undercoating? Take a look at the thick pipes (coolant pipes) if metal, are they deeply rusted? Look for seepage of coolant (either wet or as a dried up salt deposit) Look at the small tubing (brakes). Looks unrusted?

Is the sound deadening bottom pan still there? (no catastrophy if not, since in the US it is not checked at the safety inspection) Is it about to fall apart due to rust?

Body rust

The busses rust first along seams and windshield glass. This is annoyingly hard to fix permanently.

Some rust is from the inside. This shows up as unbroken bubbles. Common on the utility inlets (above) and the front lip (2nd below), where water enters and runs around the inside of the metal, then rusts though inside-out.

Anoter area is the lip just above the bumber.