b6S4 brakes should fit as should b5 S4 brakes. B5S4 brakes are plentiful and cheap to acquire... also larger & more powerful than the 3.0 brakes which are 312mm vs the 321 of the b5S4.

i was told if i wanted to do a simple upgrade, a 3.0/stage 1 kit would be the way to go.

They were a direct bolt on.