Ask yourself first how much you want to spend on the car itself...

You can pick up a nice B5 S4 2000-2002.5 for anywhere from 16k to 26k depending on mileage and what is done to the car. I would stay away from 2000's because that is the year with the most problems but if you know what you are doing and are willing to work on the car yourself that is where you can save money.

B6 and B7's are simply more exspensive because they are the newer models. I personally like the styling but find the tunablity of two turbos much more interesting that a naturally aspirated V8...sometimes.

My car is a 2000 model which went 102,000 miles on stock turbos and dynoed 260 h.p. and 326 ft./lbs of torque to the wheels the day before one of my turbos blew.
Turbo failure seems completely random however if you chip the car expect that day to come sooner than later.