check for rust, don't worry about surface rust and stuff, you wanna look under the car and check the floor boards on the passenger and drivers side, the strut towers are a very important place to check for rust, front and back. the next thing i look for on any car when i am looking into buying it, is look at what kind of a driver had it previously, anything laying in it or under the seats or in the glove box, was it some kid beating the piss out of it or was it some old lady that only used the first 1/4 inch of the pedal to drive it. since they're diesel engines, you want a diesel with good compression, so it shouldn't start too hard, althought bad glow plugs will cause hard starts, if it has good compression it should still start either way. once it's running open the oil cap and check how much air is blowing by, and compare it to an engine that you know is good, obviously another vw diesel, not i can tell you most engines will have a bit of blowby, at idle there will be a nice amount of air coming out, it feels about the same as if you blow on ur hand pretty hard but not extremely hard. anyways, watch when it starts for smoke and stuff. go to each corner and push down on the bumper and see what condition the shocks are in, look in the wheel wells for broken springs. lots of stuff to check.
i love going to look at cars with my dad, we look those things over like crazy, we're a 2 man wrecking team, nothing gets passed us haha