Buying advice Youngtimer ?

Hello together, so I just want to get a little bit less as much as possible up to 500€. A friend has got himself for almost 2000€ an Audi 100 with 150PS, for 2 years no problems with it. Did also somehow think of Audi 80 or 100 ? Mercedes W202 C280 ? What would be recommended with BMW ? It should be a gasoline with automatic and preferably not from 7th hand, you understand what I mean possible 5 or 6 cylinders. What should you pay attention to ? Where are there more rust problems ?