how long is a bit of string? . . . .
there are so many different things that can go wrong as with any car,
i cant view the add, but does it say FSH? If it has i would check to see when things like, cambelt, waterpump, starter motor etc have been changed
If they are looked after and serviced right they are great fun but as ive found be prepared to fix alot of things, not too expensive but sh!t goes wrong!
if you like the car just working and beign fine keep the audi, if you want a mkii be prepared to spend some money on wither a mint one or one that you will need to replace things on
again its a 20yr old car

but again check out the buying guide in the FAQ