I have a 2004 A6 S-Line which has the 2.7T , and IMO i like the look of the A6 better than the S4 and, you can do just about anything to the A6 as you can to the S4, if your looking for a 2000-2002 you can find a rare 6spd, and with that you could add much more HP well over 400 easy, but get ready to pay :( A6 isn't as cheap as you think. And make sure they did the 75K service which is timing belt/water pump...ect and at the audi garage its $1200 , so you need to get that done if it hasn't been serviced already, but Good Luck , and expect to pay around 9-12K ish for a 2000-2001 with 80-110K on it, we got a 2001 2.7T for winter and was only 14K with 50K on it, so look around on autotrader.com
As for mods, chip,downpipes,exhaust,diverter Valves - add big power. so look in that direction.
www.ecstuning.com
www.awe-tuning.com
www.goapr.com
www.achtuning.com
www.gmpperformance.com
www.gmgracing.com
www.vastperformance.com
www.ebay.com (has some decent stuff) :O
are some sites to get you started, Have Fun
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