V8 S4's are fantastic vehicles. A great all around car. Less of a "drivers" car than an M3, but does very similar performance numbers, both in a straight line and around the track.
My dad had a 2005 (B6) S4 6 sp with AWE non res catback, sounded amazing. Gas mileage was about 21-23 daily.
I would try to get a B7 S4 though. Just more refined as far as the suspension and steering goes. Also gets a new steering wheel, Bi-Xenons's, better NAV, and a 40/60 f/r tq split.