Hi Gordy, I think you have two options with your budget and intended use in mind.
S3 upgrade - pros are it's cheap and works well, cons are you can still stretch them too far on track as I did even with DS2500 pads in (which I think is not the best pad choice for track but great on road) and ATE Super Blue fluid. Take the rears as well as they're vented. The fronts are especially heavy.
There are also a number of VAG (Brembo) 4 piston options that you could take with the S3 rears.
Having gone the S3 front & rear route myself I would say with the benefit of hindsight that the 4 piston kit for the front and S3 rears would be the way to go. I'm not qualified to talk about the 4 piston options since I have no experience or in depth knowledge of them. There are plenty here who do though.
The items in your Ebay link look like they've been painted grey which wasn't an original colour. They also look a bit 'crusty' around the edges. Nothing wrong with refurbished calipers if done properly though.