1993 Audi S4

My dad traded me his 1993 S4 Audi, for my 1983 GMC sierra classic.

Obviously my dad was doing me a favor on this trade, expecially considering how expensive it was for gas in my truck. Otherwise the truck runs great and ive invested lots of money into fixing it.

But I just got this audi and i have a few questions:
1)It has 127,000 miles, so about how much more life does a car like mine have?
2)Anything problems these cars generally have that i should watch out for?
3)My sun roof seems to be messed up, any information on this would be nice?
4)My roof lining is sagging, ive heard this is a common problem in older vehicles, im just wondering if anyone has any advice to fixing this?
5)Its easy to assume that repairs will be alot more expensive on this audi then they were in my truck. But HOW much more expensive are we talking? For example I replaced the gas tank in my truck, doing all the labor myself this ended up costing almost $300 for the new gas tank. Am I too assume it would be more like $500 on the audi? Or even more like $700? (note: im just asking rhetoricly, the gas tank in my audi is fine)