S3 Headunit Upgrade

Whichever model you choose you'll need the following:
Fascia adapters (blanking plates) available from Audi Dealer approx £10, the back of which will need to be trimmed to fit otherwise they stick out.
Antenna amp
aerial amp
HU removal keys - Audi dealer , ebay or big Halfords/ICE dealer

On the wiring front it really depends how you plan to hook everything up. You can just buy a wiring adapter which will allow you to plug the standard RCA pre-outs into the Audi loom for the rear speakers and the sub (the front speakers are on a standard ISO speaker block). This will also give you a loud "pop" everytime you turn the unit on or change CD tracks & possibly engine whine (at no extra charge).
The preferred method is to bypass as much of the Audi wiring as possible and wire the rear speakers directly to the new HU speaker outs, but you'll lose the sub unless you buy the adapter above, (the pop is bearable when it's just the sub). Again it's strongly recommended to replace the tupperware sub with something that actually works (stealth sub or proper boot hogger), and also replace the front component speakers.
If you have the Bose upgrade then the above is all still valid but you'll cook the HU if you run any of the Bose 2 ohm speakers off the HU's internal amp.

I just ordered one of these:

http://www.caraudiodirect.co.uk/product_...ab1baed1d6f35d2

It does change to all red display so would fit in perfectly.

What unit do you have at the moment? have you not considered an Audi RNS-E DVD nav upgrade, it will give you mp3 playing from 2x SD slots. 2GB cards are available, 4GB total. Plus you get Navi, CD chnager option and TV tuner options and it matches your car.

Depends on whats needed, I'm doing the upgrades for £1000

Exactly!