These cars are built to last if maintained. Have a look at the service records, otherwise you don't know if you are getting someone else's problems.
My e30 is 17 years old, 210,000kms and the engine is still strong.
Looking to see if the timing belt has been changed, it isn't burning oil and the body has no rust.
Other then that, they are great cars.
Past: 1998 323is (E36)
1986 325e (E30)
Present: 1997 Jetta GLX VR6
2004 325iT (E46) (pops)