Most people do not go 5.1 in their car. You will find a lot of people actually just have stereo audio and split it to go 4.1 or 6.1. Ask you self, do you really think you will need Dolby Surround sound or something like that in your car? If you go 4.1 a single 5 or 6 channel amp is all you will need; or a 4 channel and a 2/1 channel.
As for power to the PC, please do yourself a favor and buy a automotive power supply with built in shutdown controller. It will save you many headaches. The 400W PSU you have now is definitely more than you need and an inverter + shutdown controller for that will cost more than a DC-DC power supply.
As for screen I have to suggest trying to think of a way to mold a 7" screen into your dash instead of a single DIN motorized. Why? Because motorized screens break, they don't look as nice, they cost more. Their advantage? They take less time/work.