100k shouldn't be too much of a problem for a mk4, but it would be best to get one that has full service history, preferably one with dealer service history.
The main things to check are:
- Cambelt should have been changed at between 60-80k (not the original VW recommendation of 120k!). At the same time it is advisable to change the water pump as the plastic impellors have been known to fail by 80k.
- At around 100k the engine ancillaries are getting a bit tired, in particular expect trouble with the air conditioning pump clutch (if fitted) and alternator bearings, but these are not difficult to replace with new or secondhand items so if they are noisy get them done before they stop you on the hard shoulder!
- If the windscreen wipers are slow or noisy then the wiper mechanism is starting to seize up, but this is easily cured by removing the assembly and regreasing the wiper spindles, alternatively a new assembly is only around ?30-40 from a dealer.
Common mk4 faults (regardless of age/mileage) are :
- Leaking rear screen washer, either into the inside of the tailgate where the water effects the interior light/alarm switch, or at another joint in the washer system behind the C pillar trim and water leaks into the boot of the car
- Dropping electric windows due to breakage of the (plastic) cable guides, but there is a metal repair kit available and there is a good chance that some of the cost will be covered by VW as this is a known problem.
- Faulty MAF (Air Mass Sensor), which can cause a flat spot in the engine power or poor fuel economy (note this does not necessarily throw a fault code in the ECU). These sensors were extremely expensive (?250-300) but VW have recently slashed the price to under ?100.
- Faulty boost valve on 1.8T models, identifiable by a fluttering sound when you lift off the throttle, but these cost less than ?20 to replace with an OE part or about ?80 for an uprated aftermarket part.
