Hi and Welcome,

First off these are great van's (I currently own 3, one in the UK and 2 here in Aus) if they are looked after and if you can do some things yourself.

As Sunny said, rust is pretty common around the windscreen and can be repaired if needed.

A few of my own findings with a 2003 tdi auto:
Now, the auto box is a week point if not serviced properly and by all accounts even major VW service centres were unaware of the 60K service schedule of the box. There are no specific indications in the service book either! Auto box rebuilds are expensive, I paid $4.5K for one although it was in regional QLD (Luckily I only paid $500 for the complete van).
Parts in general are expensive unless you can source from Europe and labour can be from someone who knows these vans.

As for the rest:
Dirty and low oil means the last owner wasn't too careful, oil leaks probably common for the age/kms (rocker cover gasket and main seals would fix 90% and probably have never been done). The same could be said for bushes etc., basically coming to the end of their serviceable life and time to get them replaced.
When you say the engine is noisy under load is that knocking or induction noise? The air intake sits directly in front of the driver so you can really hear the turbo induction.

If I was buying I would be getting it for a very good price then giving it a major service including all drive belts, cam belts (x2) if no evidence of being changed around 120K, auto box oil as soon as you possibly can including a clean out of the auto box sump pan, renew all bushes and perished rubber items required.

Once done these are great weekender vans (just look at the following in the UK!) and are generally reliable if looked after properly and it should go on for another 200K Km without too many drama's.

Good luck if you decide to bite the bullet

MM