+1 on getting the xenon's - they make a huge difference.
We've had ours for over a year and a half now and love every minute of it.
Check out bavsound for upgrading stereo system.
I would recommend to get the iDrive system, but it's not mandatory - I just like to listen music on a USB stick, and scrolling through thousands of songs one line at a time is a pain.
If you're going to be sharing the car with another driver I would recommend getting comfort access - makes it feel like the car is actually yours (automatically sets electric seats, mirrors, radio stations, presets, etc).