Good enough cars standard...well made, reasonably swift...but dull as dishwater to drive.
But, they can be made to fly.
Don't believe all the horse-feathers of 275-280 BHP with a re-map...it's utter crap.
You can ger 275-280 BHP/330 lb-ft out of them on a K04 - I did - but it will cost you thousands...many thousands and not a Stage 1 map as many idiots will tell you.
Brakes are woeful...you can kill the standard brakes on a backroad.
LRC Brembos sort that.
Standard suspension is even more woeful than the standard brakes - Bilstein B8 dampers, H&R springs, Neuspeed ARBs and Forge/KW rear tie bars sort that out...but won't leave you with much chance from £1500-2000 fitted and set up.
But it will at least corner like it should.
They will eat suspension bushes when sorted though...and you really need 18" wheels to make the best of the set-up.
100 cell 'race' cats are a nightmare - they struggle to pass the MOT on emissions with those on. 200 cell are better.
If you can find one with any of the above already fitted...you are onto a winner.
I had mine 3 1/2 years and it averaged around 25 MPG...but made 275+ BHP and 330+ lb-ft for most of it, was utterly ballistic point to point (easily left my 911 sitting on a back road)...but was just dull to drive.
There wasn't too much left standard on mine...so I have a good idea what works and what's internet myth...get in touch if you want specifics.
Mine was an AMK engine...makes no difference when mapped, as already said.
Just a note...it's like driving a diesel once you really start pushing the K04. You can run 1.8 bar peak boost, and 1.3 sustained with good enough ICs...but that's over 90% duty cycle on the injectors and the EGTs are rocketing by then. But, with the traction of a well set up S3, you can use 320 lb-ft daily, on full throttle, easily....by 330 lb-ft it struggles and I found the car actually slower point to point, so swapped back to a standard N75, back down to 320 lb-ft and stopped fighting the car.
It doesn't drift or power slide naturally...but by f**k it grips and goes.
Good cars...really very capable once sorted (the lad I sold mine too used to bait M3s and WRX STIs at wet trackdays) but ultimately little fun to hussle hard...just a typical Audi - efficient, but dull.
I'm biased, but I'd say Nogaro Blue would be the best choice, with the blue/black Alcantara/leather interior, LRC brakes and BBS wheels...
Just like this one in fact:

It's still about on audi-sport.net somewhere...on it's 4th or 5th owner...still going strong.
I wish I'd not sold it...it was probably the best road car I have ever had. Subtle and effortlessly f'in quick wet or dry.
I'd have it back in a heartbeat.