I maybe able to answer a couple questions...

1. Stick with the newer Visteon ULF's. 552 is obviously good, 387 will likely work well, further you go back in software the more likely you will have issues with your phone. I assume you've seen: http://www.xoutpost.com/showthread.php?t=9565
You may be able to use a combo Assist / Bluetooth module, but it's not clear if you need a seperate SES box for voice recognition.

2. Yes, you will need to remove your Motorola Assist module

3. Visteon ULF has SES (voice recognition)

4. I don't know this one.

I do have vested interest in your questions as I'm selling an X3 kit on E-bay. See the classified forum for info.
Cheers.