I got mine from the dealer. I think I paid ~£125 each with the VAT a few years ago. I am aware you can get a set for around £400 online/ebay, but do some diligence to make sure they're not fakes!
Yeah sure, you can put S3 ones in if you have an Edition 30, otherwise stick to GTI ones if you're not planning on going KO4. The RS4 valve is a good idea if your map requests 130bar. The OEM valve starts to open at around 127 bar I believe, whereas the RS4 opens at 137 bar.
I have a full kit of parts to clean the valves if you're interested. Injector seal kits x 4, manifold gasket, a can of 'Carbon off', a kit of brushes and the correct tool for fitting new PTFE seals to the injectors. I bought it all ages ago to do mine, but I decided to get it walnut blasted instead. I've been meaning to shove it all on ebay for a while, but would rather a member on here make use of it