I will try to backup a couple steps to help simplify things for you...

First, these are the only parts you need to get Bluetooth working with your iPhone 3GS. It does not answer your question about a charging cradle, but that is totally optional. This gets you fully functional Bluetooth.
84.10-9 195 455 Bluetooth Assist TCU module (Or another BT-enabled TCU from the prior list)
84.50-6 928 461 Bluetooth antenna
61.12-0 137 616 Bluetooth antenna cable

If you want to get the charging cradle also, you will need to add the support base, the snap-in cradle, and the 3GS snap-in adapter you reference in the original eBay post. I am not sure which base & snap-in cradle parts are correct for the X5 with a TCU-- more digging to come there. However, all of this is optional and you can add it at any time in the future.

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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