Good luck with that one!
I don't know how tall you are, but I gave up on fitting seats in my E46 sedan pretty quickly. I also have twins and there was no way I could fit a rear-facing seat behind my drivers seat. If you can wait until it's time to turn your kids around, there are some relatively compact seats out there like the Radian. It's narrow, but the recline angle when rear-facing will make it impossible to fit. Pretty much any of the convertibles will fit when forward-facing.
Best bet is to go to a store and try a couple out for yourself. Or head over to http://www.car-seat.org There are a couple of E46 threads over there.