Unless you are going over 150 w per channel
or running tweets on separate channels no need to change
the existing wiring. Any wire that will handle whatever
amount of watts per channel you want will work .
Please read
http://www.e90post.com/forums/showthread.php?t=641323
I don't know if the Alpine kit is a good base for and upgrade. Most that do the Alpine Kit don't go any further. There are a lot of people on here that believe spending money on rear speakers is a waste of resources.
Contact Technic for the Harness be sure to tell him what you have
and what you want to accomplish.