At 80000 Make sure the cambelt has been replaced. Also check the brake discs and pads for wear. I should think they are as good as gone too if not replaced yet.
BTW are you looking for a 2.0tdi or the 1.9tdi?
The 2.0 is prone to cracked cylinderheads, but according to most german long term tests of the 2.0tdi block it's not a dealbreaker. The cracks are very small and seal as soon as the engine warms up. Also you can only check it when taking the engine to pieces
The 1.9tdi has been around since the beginning of time so there are very few problems with it. Check for juddering when stationary, can be a faulty flywheel, I think it's called, and check for clutch slip. The 1.9 has a weak clutch and if the car has been chipped I'd do a couple of full bore starts to see if the clutch holds up.
For all models check for uneven tyrewear especially on the rear. You'll hear a rumbling noice at certain speeds coming from the back. Would be a shame if you have to buy new tyres a few weeks after getting it.
That's all I "know" about it
good luck!