Dude, I'm honestly no expert but, like yourself I expect, I got no straight answers from my dealer and turned to the internet for support
Here's my understanding.
If you want to stream music and control your phone via iDrive, you need enhanced Bluetooth which is available separately or comes with the NAV system. You're then responsible for charging your phone yourself.
If you want to use a standard iPhone cable and plug your phone into the USB socket in the armrest (or glovebox) and do the same thing, then you want USB Audio prep thingy but as far as I can tell, they effectively achieve the same goal, the only difference being that you have to physically plug in your phone for this option. Oh, and it charges obviously. Another plus to this option is that you can buy a cheap 64GB USB stick, copy a shed load of muzak onto it, plug it into your glovebox and forget about it.
If you want to stream internet radio and stuff and use MyFace and Twitbook then you need Mobile Application Preparation. This automatically adds USB Audio prep. There are also a few other fun apps available but not many. One "big" plus of this is that you can turn your iDrive screen into something that looks just like an iPod screen when you're playing music.
If you want to stream video (?) then you have to have the full media cradle but in order to have the full media cradle I believe you have to have Apps and consequently USB Audio. The media cradle also means that the phone uses the car's external antenna for making calls, theoretically giving you better reception.
I would say that if you're not getting NAV and you want maximum connectivity from your phone then get enhanced Bluetooth and USB Audio as a minimum. If you want the cradle then get the Mobile Apps thingy. I covered all bases and just ordered the lot along with Pro NAV, partly because I wanted maximum functionality from my phone but mostly because I'm a sucker
Please don't take this as gospel though - by all means use it as a guide to help you in the right direction but I don't yet have my car and when it comes, I'll have every media option so I can't really say for sure what does what
Hope this helps somewhat though!