A KKL cable, from eBay at around £10 and VCDS Lite, free download from the Rosstech official website (and a Windows XP -10 PC) is all you need to check out a prospective purchase.
Last time I looked, Carista did not support the A2. OBD Eleven does support the A2, and is very convenient as there's no wire between the OBD connector and the phone/tablet it runs on, being Bluetooth. Quite expensive though, and is biased towards newer VAG models. VCDS, having been around for a long time (it was called VAGComm to start with), supports all early VAG cars and engines, A2 Mk1 TT, etc. The technical support available from Rostsech, and the community around it is second to none, where as getting a useful response from OBD Eleven is not easy..
There is a small set of error codes which most motor manufacturers signed up to, and support, and most readers will read them accurately. Then there is a much larger set of codes which are manufacturer and engine specific, not all readers can read these codes accurately., as the helpful label that gives a brief description of the fault idoes not come from the car, it is pulled from a directory of labels within the reader, and attached to the appropriate code.
The owner, and developer at Rosstech, Uwe Ross, still answers tech questions.
Mac.