Welcome to the forum! There are a couple of folks undertaking similar projects to what you are proposing. Fargo has put a carputer in his car, and done some impressive fabircation work to install his screen. Brettster has a carputer installed, and has mounted his screen in a similar fashion to that which you propose. I'm installing a carputer, and using the OEM satnav housing to mount my screen with the PC in the boot. For more info on audio, you can use www.talkaudio.co.uk and to get information on the xenarc/lilliput debate and find out all about specialist carputer software go look at www.mp3car.com
For radio, have a look at the Dlink USB radio device. This will work with Frodoplayer and others, allowing you to have radio on your carputer.
You can maintain the 6 channels, you'd simply need to have a six channel amp (or 2 channel + 4 channel)
If you use an OPus PSU, you should be able to shutdown or put your PC instandy, talk to Brettster/Fargo about this, they have their carputers up and running, and I know Fargo uses the Opus, and Brett puts his PC in standby frquently