If you are gonna use the old collared nut then you may as well use the spring top plate too as the whole assembly will be in bits anyway, the collared nut is the last piece of the 'puzzle'!
I used completely new parts, built everything up and then fitted it in one go at a later date. It means that the old shocks can be removed and kept in 1 piece should you ever want to go back to standard.
I have a 1.8TS so the suspension setup at the rear is very different to your S3, I can't help you there as I have no experience with S3 suspension. The front suspension is similar (with the exception of the location of drop links etc).
Found my old receipt btw:
Rubber Stop (front) - 1JO 412 303 - 7.95
Bellows (front) - 357 413 175A - 7.07
Spring plate (front) - S1JO 412 341 - 5.18
Bearing - S1J0412 249 - 5.60
Strut mounting - 1J0412 331C - 9.10
Threaded Bush - IH0 412 365A - 1.49
Total - 72.60 + FAT! And that's with a good discount from my local TPS branch through someone elses trade account, I imagine that the parts would have been in excess of £100 for the front alone otherwise!