I wouldn't even bother with the rear.. put your money up front where it counts, id do a nice 6 inch component set with tweets in the door pods. And if you want bass you can buy a 10" sub with an amp for about $400 it will blow away the 6x9's
I have a few e30 parts for sale, updated often, discounts for local pickup/delivery. Check out my eBay
account for details.