With a 12 or 15 you will be rattle city in that small car which you claim is already rattling (atleast you won't hear the road noise). Also, without upgrading the deck, speakers, wire, and interconnects just throwing a sub in there will be too overpowering. I would stick with a 10 (or pair of 10's depending on what music you listen too) and use the money saved to get a nice set of component speakers to compliment it.
I think your first step should be to isolate the rattles and tear out the interior to dynomat. It will greatly improve the sound. Also an alarm to protect it all
Everything you want can be purchased at Crutchfield. I personally favor pioneer and kenwood, but I have been out of car audio for a long time.