I want to install an aftermarket in-dash GPS navi / DVD / CD in my 2002 Mercedes ML. What are my options?

I have a Mercedes ML 500 with an in-dash CD changer, tape player, navigation, screen, etc, and the amp is under my seat, and there's a 6-disc changer in the back. I also have screens in the headrests, and a DVD player under the back seats, but I don't even know how to use it, or maybe something is wrong with it. I also have an electrical outlet in the back that works fine. Anyhow, I am having problems with the sound cutting out, and I think it's mostly due to a problem with the 6-disc changer. I read about it online, and it seems to be a common problem that might be best fixed by buying an after-market in-dash system. The thing is that I want one with the GPS navigation included, as well as mp3 connection, blue tooth, TV, and DVD, and I want it to be able to control the screens in my headrests. I don't want to pay a huge price, but I'm aware it's going to cost. My goal is to be paying less than what I would pay in the long run to keep fixing the sound problem I have. I don't know what brands to look for, or even exactly what my options are. Does anyone here have some useful information? Where should I look? Are there any websites that will offer aftermarket systems (fully loaded of course) that will be compatable with my Mercedes? Is there a certain kind of part I am looking for, or can I put any system into my car? Mercedes MLs are kind of weird electrically, so I don't know. Please help!