Your 2007 328xi has the same HiFi system as my 2007 328xi touring. It's described here:
http://www.e90post.com/forums/showthread.php?t=407085
I would be really, really clear on why you want a BitOne. If you already have Bluetooth and iPod, and aren't using the BitOne for that, then it has to be tuned by one of maybe 100 people in the country or it's a waste of money.
I take issue with your characterization of sub boxes - you don't want one, cool, but that's not what our systems use them for.
The main reason to make door panels is to get more midbass out from under the seat and into the door. Is yours a sedan or coupe? If you have a sedan, I'd go with a good 4" for mids whether you go with panels or not. If you have a coupe and don't want a box, consider Jehnert door panels.
If you're talking about going to EAS for audio (I was not aware Tom was doing audio and last time I talked to him he indicated it wasn't on his radar), I'd throw my hat in as well. I wouldn't attempt to do many of the things EAS does, but in all frankness, we are the premier BMW audio upgrade experts on the West Coast.
If you like, send me a budget range to hit and I will happily send you some different approaches that have strengths and weaknesses highlighted within that range.