Hi,
Firstly what car do you have? im driving a mapped Ed30 and have these calipers on the front, I wouldnt bother with the rears, even during ring trips and track work its very rare that the brake ballance feels wrong, or that the arse end steps out.
If you bought the S3 calipers I wouldnt bother refurbing them red, if you did its a simple enough job to pain the fronts, as you can split the calipers. The rears are a lot harder, hence why I wouldnt bother buying/fitting/refurbing them.
I doubt you would need new discs, they range from £75 is each upwards, id fit new and better pads say ds2500's there about £160 a set.
Fitting will vary, you will need new stretch bolts to mount them and go with some braided hoses since you are having brake work done anyway and you dont know the history of the old ones.
Oh and change the S3 caliper clips to the "R" type ones (see my for sale ad lol) and sell the S3 clips on the audi forums for a bit of profit..?
steve