Stereo installation:
You can purchase a converter to change the double din to a single din, there a few manufacturers although I'm not sure who does them - a few people on this forum have them so they should be able to help you.
Beep in reverse:
Just used so that you know the car is in reverse, first and reverse are close together so it can be easy to make a mistake when you have just got the car.
TTE Super-Charger kit:
3900GBP with fitting http://www.tdi-plc.com/is200.html
Advantages: Increased power & torque
Disadvtanges: Increased noise, does not add much to resell value, increased insurance
Warranty: 1 year
Keiran