For more modern VW's, Skoda's, Audi's and Seats do look for a few things.
High mileage on the car doesn't necessarily mean it may be a lemon, it is totally dependant on the cars history. If its been serviced by a main dealer you can generally ask for some service history (like any technician notes)
Example:
This is some of the most common issues with higher millage VAG Group cars, shuddering in the clutch is generally a mass fly wheel (€600 from VW, ive had the shudder from about 135,000 - 240,000km), coolant bottle contaminated is another one, its a cheap part from VW but causes loss of coolant should it be contaminated or ineffective. Other general wear items on diesels would be glowplugs.
Tips though if you are testing a VW and perhaps the price seems a little too good.
- Check Coolant Bottle colour, if its a darkish colour it may be due replacing; This effects the pressure in the coolant system and will cause an airlock, drive the car and get it warm and whack the heating on as hot as it will go, if you notice any "cooling" effect after a few minutes you may have a failing coolant bottle.
- Get someone else to sit in the car and lock the wheels in one direction and ask them to slowly let out the clutch, if the wheels start "shuddering" and/or the clutch feels like its shuddering a mass flywheel is almost due. In addition to this leave the car idle and listen for any odd noises which may be gearbox related, alot of the time you can hear a slight rattle indicative of a mass flywheel, its better to do this with the bonnet closed.
- For the 1.9TDI (5 Speed) engines start off in first and at about 2500 rpm change to third, if you get more consistant power in third it means the airflow and turbo is working as expected, if you get nothing but then get a sudden belt of power (and i mean a belt of power) it can be an indication of a faulty airflow meter/sensor.
These are very specific checks for the above engine, it is good for most of the VAG group from 2002-2012.
For the newer 1.6/2.0 TDI engines, at the first sign of a rattle divorce it.