I'm super new to the Audi S4 info but I've been actively shopping for one very hard for about 3 weeks now and have talked to a lot of the shops working on them and a lot of people who are selling modified ones and I'm hearing consistently that once the cars are chipped, depending on how often you drive it hard, the factory turbos go out in 10,000 miles or so, then that's when it makes sense to upgrade to the K04's or something but it's costly. Once you upgrade the turbos, you have plenty of power, just run good gas. Then you need RS4 intercoolers, bigger injectors, depending on how you drive, you might want to take a look at the brakes. I could go on and on. It takes $$$ as usual. That's why I'm looking for one that already has most of the expensive stuff because sellers can't ever get what they've invested in the car back out, means savings for a buyer.
I think I've found mine, just got to get rid of my Nissan first. Which is currently listed in the “cars for sale” forum.