Look for water ingress into the cabin as the roof and it's seals will now be over 20 years old. If you need a new roof this could cost you £2k at a guess.
Mechanically it should be ok as long as service as been kept too. If it needs a cambelt and water pump change then factor a bill of £500-£600.
Also speaking from personal experience of running a an old Audi almost daily, you may need to factor in high repair costs. My a6 has been owned by me and my father before me a d it has been maintained regardless of cost. This year already it has had it's routine service & mot, plus two boot struts, a window regulator a d us going in next week for the central locking and alarm to be looked at.
My a6 is sentimental so I won't get rid of it, but I do have a newer car for daily use. It's not all bad though, for example my a6 completed 2,253 miles to abd from Scotland whilst doing the north coast 500 and didn't skip a beat once.