A lot of the hardware you seek (if not all of it) can be found in the store section of this website. Various threads deal with the pros and cons of all the hardware you have available.
Incorporate OBDII -
http://www.mp3car.com/vbulletin/engine-management-obd-ii-engine-diagnostics-etc/
Power ON and Power OFF ability with Ignition
http://www.mp3car.com/vbulletin/power-supplies/
All forms of DVD playback/Video Playback
- Just make sure you have a good DVD player with good enough software and hardware to support it.
MP3 Playback - LOL, this one goes without saying. A good audio player combined with the right front end software is essential.
Decent Sound output
- A good head unit or amplifier is essential.
Also if I get an AMP would I need to keep my head unit? - You wouldn't NEED to, but it would require some knowledge of wiring audio. If you are running the sound card directly to an amp, make sure the sound card is high end. Generally, the factory sound card isn't up to the task of competing with a head unit. Or this is at least what I have heard from other users.