>but does anyone know of anything in particular about these that is very
>problematic/expensive that i should be wary of?

Check the condition of the convertible top carefully, including listening for air comming in around the seals at highway speed and leak testing it at the carwash. (unless it's cheap enough to be worth putting a new top on)

If the A3 Cabrio is similar to the A1 Cabriolet, there are numerous adjustments to make the top fit tightly at the windows, but they can't compensate for worn out seals (or holes!)  Mine needed a new seal where the top fit against the windshield.  Even with the latches adjusted tight, it would whistle above about 60 MPH.  It also leaked water onto the seats, due to a variety of rotted/worn out seals, window problems, etc.

Bill