All of the above, but you do not have to tackle it right away. Based on your comments, this is the priority that I would recommend.

1. BMW TV module - This is very cool, but you probably won't use it that much. The only reason that I put it first is because it is a prerequisite for the backup camera. (There are other video source modules that will support backup cameras; however, if you are interested in the BMW TV module, you might as well start with the best.)
   
2. Backup camera - I too have a primary safety concern. There is nothing like being able to see exactly what is behind you, especially in an X5. PDC and a backup camera together are the ultimate combination!
   
3. MP3 Player - It is very easy to install an iPod adapter or one of the generic CD changer aux input adapters. Or, you can simply install BMW's Aux input to your radio and use that. I have my iPod running through the radio's Aux input, and I love it. This solves the problem of the CD changer and listening to anything (books on CD to whatever...).
   
4. Satellite radio - It sounds like you have a decent satellite radio solution that works for now, but you probably do not use that much. When the XM subscription runs out, I would install the BMW Sirius kit to replace it. If the cosmetics or audio quality get to you, then bump it up the list.
   
5. CD Changer - I would blow this off. If you have satellite radio and an MP3 player, you will never use a CD changer.

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SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input