VP Electricity "musicar" is the guy to go through who sells the drop in kits.
eveyone on this forum will swear by the SWSs. however, you'll find a small minority of us, im one of them, who will tell you the Jehnert XE200 underseat subs will give you a clean, full sounding base. i think you'll find when set up properly you will not need a trunk sub. i was on the verge of doing it, and since i had my amp tuned properly i feel like i can def get by without one. The Jehnert subs play deep notes really really well.
I used the SLCs for about a year and was really really happy with them. they are very balanced and full sound without being too harsh. Ive heard good things about the Morals (spelling?) as well. I ended up recently switching to Rainbow Profis that have absolutely blown me away with how well they sound playing all types of music.
The Zapco amp i have i feel like it does an awesome job, and its user interface over the PC, from what little bits i saw at my installers was extensive. this amp pushes the drivers very well.
There is a user here who recently bought a level one upgrade from VP and successfully installed it into his 3 series himself recently.
Def get in touch with him, Kansas83 his page is here
http://www.e90post.com/forums/member.php?u=108574
technics harness is a must.