Audi A4 convertible is a really nice car, 4 seater and decent boot so practical as well, they have excellent build quality as you would expect, hold there value and not a problem to sell on in the future.
I would steer away from the auto box versions unless they have had gear oil and filter changes @ the 40,000 mile stage in which case they should be ok. The handling is ok, but remember it is a cabrio.
1.8T engines are good with plenty of tuning to be had, servicing is not a problem, 3.0 v6 is thirsty but has Quattro iirc so has the v8 which are rapid but expensive to run. It has a folding soft top which is properly waterproof but just make sure all seals are clean and soft. All in all a decent reliable practical car which should give you no real problems and plenty of fun when/if the sun is out.