i don't have any experience in installing or removing the head units, but you might want to ask woobie about this.
i'd look for an alpine unit. also, the alpine CD changer will work as well. there is an mp3 player that is compatible with the original head unit, but i can't think of where you can get it. i'll do some searching on this.
the 10 speaker is the standard audio system, and the 12 speaker is the upgraded HK system. look at your speakers, especially on the doors. if it says Harmon/Kardon, then you have the 12 speaker system. another way to find this out is to look in your trunk. if you see a black box up above, that's your HK sub/amp. if you do not have these, then you have the standard 10 speaker system.