If your are considering a six speed the absolute must have is C7. It includes a short throw shifter, which is much better than the standard shifter. In my opinion the Panorama roof is a must have. If you live in an area where it gets cold, they heated seats are great. The bi-xenon headlights are a good buy, too. If you want leather, but the car you are looking at doesn't have it, it can be added for $1300 to $1500 at a couple of places like Classic Soft Trim or Katzkin.
An aftermarket headunit cannot be connected to either the CANBUS or the D2B fiber optic loop. Without CANBUS, you can't use the steering wheel buttons to change the volume or stations on the radio. Without D2B, you can't use the Bose amp, an integrated phone or a factory cd changer.