John, that post is huge. You say whats a search button, i suggest you find it, spending some of your own time to research your questions before asking always gets you better answers.
I'll answer your volume question. That volume is independent of your sound cards master volume as set by MC/ME. Leave it on nearly full and control the volume with your software.
Your HD is going to struggle, thats a hell of a lot of vibration, mount it as far away from yr subs as possible.
Your composite screens will struggle wth small text/GPS but should be fine with DVD.
You should consider the quality drop in VGA-RCA conversion. You should also consider the fact that by using dual monitor, or monitor/TV in your situation you can display different applications on different screens, i.e. media car on yr VGA screen, DVD on the headrests.
1 200W PSU will suffice. If you use 2 u will need to do smoe solder/wiring to get it to turn on at the right time. ATX PSU's require a pulse from the motherboard telling it to turn on the main rails.
As for your audio output, get a creative extigy or similar 5.1 sound card, m1000 onboard sound is known to be iffy. This will give u the outputs u need.
Audio input, you will need to use a 3-->1 mini headphone adapter. Or get an audio selector. I think the adapter will suffice, pretty easy to make one yrself.
Thats all im gonna answer for now. Do a searcha nd if yr still struggling then ask. Not before.