Six hours is a bit steep but the rear calipers on an S3 are a pain to remove and require a M14 spine bit. For a garage i'd certainly say 3 hours 'ish. I have the Pagid rear discs on mine and they are basically what Audi use anyway. The disc price can be a little steep for the fronts but it seems like prices may have come down. My rear Pagids I think were £109 for the pair.
If you have the time i'd certainly recommend you try it yourself other than standard tools all you really need is a breaker bar, M14 spline bit and strong arms unless you have a two poster lift as getting some leverage is a little difficult!