Hi,

first of all congratulations for chossing to buy an V8. It is a very reliable car providing a lot of fun to the driver when maintained and taken care of regularly.

The most important thing to look for is the time belt changing interval. For the 4.2 it's all 120.000km, for the 3.6 all 90.000km. When this is done by Audi, it is quite expensive (ca. 1200€ -1300€ I guess). Keep that in mind, when buying a car and this has to be done. Try to find a car with a full service history. This might be a bit complicated because even the youngest V8 are about 13 years old right now.
Rust is no real problem with a V8 apart from the area behind and above the rear bumper. Look between the body of the car and the chrome border. Chances are that you'll find some rust there, since this is a typical problem. Moreover it will be hard to find a V8 with no rust there ;-)
Personally it would try to find a car with low mileage, but it is no problem to buy a car with even more than 200.00km when it has full service.
Check all the electronic gadgets inside the car. Ask for the oil consumption. Normal is up to one liter per 1000km. Another thing is the steering. Check that it does not leak any liquid.
These are the most critical and important things to take care of. Besides those, there are the normal things when buying a used car like shock absorbers and the like.
The normal price for a V8 reaches from about 3500EUR up to 5000EUR, depending on the condition. Take a special look when the offered car is way cheaper. It is said by many drivers here that a V8 always costs about 5000EUR, whether this is the price for a good V8 or the price for a "notsogood" V8 plus repairs.
When you are looking for a 4.2, it will become very hard to find one from 1990, since it has been invented in late 1991 ;-) But for the price this does not really matter, because the year of manufacture is not really the crucial factor.

Hopefully this helps you when trying to find a good car.

Good luck
Roman

P.S.: More information can be found under "Kaufhilfe" in the navigator on the left. Since it is only available in german, you might have to find someone who is able to read it.