Along with the ET, you'll need to know the rim width, which should be 7J.
The A2's 17" 9 spoke wheels look stunning, which is why so many owners acquire a set, but there are several things to be aware of before buying a set.
The wheels are very heavy, and all that unsprung weight will test the quality of your suspension.
The wheels are not that strong, and are prone to buckling. If you have lots of potholes round your way, they're probably best avoided.
Many sets offered for sale on a certain well-known auction site are buckled, although the sellers won't advertise the fact. It's best to buy these from sellers with a solid reputation, and you should always check the wheels for yourself before handing over any money.
If I was wanting a set of 17" wheels, I'd be placing a wanted advert in the market here. One or more of the ethical breakers here will likely have a set available which will come ready to use.