Well, from what my installer tells me, it seems that the radio system is integrated into the total idrive package and its not advisable to change the head units.
Saying this, your next solution would be to use your original head unit but get a line converter to get some rca signals so that your processor or amps can use.
Just changing the speaker would give you some improvement but not much as the power from the original headunit would not be enough to really power them.
A simple setup would involve running something like a LC6i to get your rca signals, then an amp to drive your new door speakers and probably a sub in the boot.
This would be the most simple upgrade and after this, the sky's the limit.
Price would depend solely on how much you are willing to spend.
I would advise against changing the under seat woofers but instead recommend taking the $$ and putting in new subs in the boot or parcel shelf.