I gather from the SMG and auto S4 reference that a manual isn't an option? If it were, and E39 M5 would be more enjoyable than the S4 while still retaining some M-car sportiness.
The idea of a used 300c or Magnum as a business car would also be preferable to the Audi. My wife drives an A4 and I've driven a few B6 S4's, and they're great for incliment weather or a torquey car, but I wouldn't personally replace an M3 with one.
2000 IR/Bk M Coupe