What interior does the car have? Volkswagen offered three types of upholstery; sport cloth (nice, heavy woven fabric), velour (ugh, lint trap), and leather.
I had the sport cloth in my 2002 GTI, had no problems using a mild solution of Woolite and water - 6:1
I have heard nightmares from velour owners because the velour is like velcro in the VWs for some reason, it picks up everything. You should be able to treat it just like the sport cloth, just rinse well.
My wife and I now own a 2003 Jetta and a 2003 GTI, both have leather interiors. I clean them with Gold Class leather cleaner and conditioner. If the seats are really dirty, I'll use the previously mentioned water/woolite mix to clean them.
The floormats are pretty easy to clean, and if ours are heavily soiled, I'll hit them with the pressure washer and then vacuum out the water with our Shop-Vac. The actual carpet in the cars is pretty thin and collects fur and grass which can be tough to vacuum out if it gets worked into the fibers. I haven't had any stain experience with the carpet, so I don't know how tough or easy it is to clean.
The dashboard in the Jetta is a nice matte black plastic, and looks best when you just wipe it down with something like a non-carnauba QD. I use Eagle One and get nice results. I haven't seen what the Jetta dash looks like with 303, but would like to try it some time.
Also, the headliners in the cars with sunroofs gets pretty dirty if the owner drives with it open, so be sure to vacuum the headliner and check it for stains too.
Oh, and just as Tom said, NEVER listen to the VW service advisors. I have yet to meet one that I would actually trust with my car, that is why I service them myself.