http://www.e90post.com/forums/showthread.php?t=641323
for bluetooth and usb/ipod
the known to work popular units are the dension gateway 500
(you need and add on module for bluetooth with that one)
and mobridge. Both have websites .
The Dension seems to be the one most used here.
There is the factory add on but it just does ipod no usb
and costs the same as the other to or just about.
Because Most Bus is used to connect add on s there are limited
options for these add on's
If you read the above thread carefully it should answer most of your questions. There have been quite a few from England on this forum doing mods lately so you might be able to see or listen
to some of there systems and get and idea of what you want to do. mob17 and makkan00 I think are two of them.
The MusicarNW kits are the easiest (+ most complete and plug and play) way to go but they come from Portland Oregon USA.