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
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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!