Adding the deck is absolutely pointless in an E46.
The speakers all have retarded impedance ratings, so you can't run them off the headunit amplifier (some people have done it, I wouldn't recommend it), so you're looking at replacing all of the speakers as well as the deck. After that, you'll still have no bass. Now you have to get another amplifier and a subwoofer.
You could just add an amplifier plus subwoofer and tap the signal off the rear speakers, and that would give you the bass you're looking for. If you're going to do anything first, go this route.