I live in Ireland and have a 2003 2.5 TDi from new. It has covered 135k miles. Besides the normal oil services every 10k miles or so (around €300 euro from a mail dealer), I've done the following:
- Two front air springs failed and replaced (one at 80k and other at 100k miles - €500 each from a dealer. There are cheaper alternatives from Arnott Industries in the US that have received good reviews.
- Aircon radiator fan failed - €300 from dealer
- Front pads are expensive - around €120 for fronts and they last around 20k miles. You can get non OEM cheaper which some say are better. I was lucky enough to own a 1996 911 at one stage. The pads for the 911 were half the price of the Allroad
- Control arms (top and bottom) and track rod ends - started to go around 100k miles. Budget around €1,500 to have them replaced including labour at a specialist. There are non OEM replacements available that are cheaper and have received good reviews.
- Shocks - I think mine are due as the ride is getting very bouncy. €€€ from a main dealer, but Arnott have Bilstein replacements at $799 for all four not including labour. May try these.
- From a diesel perspective, oil leaks are not uncommon when the engine ages. I have one that looks like the sump gasket that I need to get checked. Hopefully won't be too expensive to get fixed.
- As was mentioned, timing belt on diesel needs to be done every 80k miles - around €1k from a dealer. Specialist will do for around €600.
If you are buying, make sure you buy a good one (goes without saying). Get the car checked by a mechanic if possible. It's not unusual for a car to get put on the market just before some major work like a timing belt is due. Check for this and use it to bargain.