That's the standard solution.
Least expensive known to work .
I think Technic can sell you speaker adapters
and factory connectors to.
Don't know if he does the tweeter pod mounts
you may have to order that from BMW.
I suppose you read this to get the info
http://www.e90post.com/forums/showthread.php?t=641323
You could use better speaker but that would be
a lot more expensive (Jehnerts, Morels)
I like it when people do there homework