Depends on the car.
On an Opel: Steering rack, shocks, suspension. By Suspension I mean the springs, steering rack and also the ball joints and stuff like that. Maybe if the guy drove like a complete twat the CV joints may need replacing. Gearbox and engine will survive just about anything.
On other cars there will obviously be other things to look out for.
My opinion: don't buy an off road vehicle with non-OEM air filters (IE. K&N and what not). There is research online that their filtering properties are lower than a paper based filter (The OEM kind). If however they are using a cold air intake (that is a pipe that goes to the front of the vehicle with a cone air filter attached) and the cone filter is the paper based type check the inside of the pipe and see how dirty it is.
A clogged paper air filter isn't really anything to worry about in terms of engine damage, it'll just lead to reduced performance. So long as the paper based OEM filter is there, it's fine but you might want to clean out the airbox when you take ownership.