That looks like a good start to your setup. You are missing a GPS antenna/software, Bluetooth dongle, and wiring. You'll probably need a USB case for the slotload DVD player also. I think they have them in the MP3Car store. And you'll need an OS.
Headunit vs. No Headunit:
You need to decide if you can handle everything through the PC, or if you want the ease of still having a headunit to play CD's and listen to the radio and have the PC for when you want MP3's and movies.
To keep running with your headunit you will need to check and see if it has some sort of aux in that you can run music too. If not, you would need to either buy an aftermarket one that has this, or use an FM modulator (not reccomended).
To get rid of you headunit, you would need to buy an amp. Once you get an amp you may as well get some aftermarket speakers because stock ones tend to run off such low wattage that they won't sound good running off an amp. Once you have an amp and speakers, you just run a line from the Sound output of the PC setup to the input on the amp.