Some nice plans you got there! Would like to see that install when it comes off the drawingboard and into your car
Think you've covered pretty much of it. But there is something, how are you going to turn on your CarPC? And how are you going to regulate the voltage? On mini-box they say that the pw80 this already does "11v-30v IN" but I can't quite find if the input should be constant (car environments are pretty harsh on electronics and produce nothing near a constant voltage). Anyway the ITPS module should do both. More info on the mini-box site.
The alternative for the pw80 + ITPS is off course an OPUS. Endlessly discussed on this forum. So if you do a you'll find plenty.
I myself have a EPIA M10000 + pw70a + ITPS, even got the same power supply for indoor use. Works like a charm.
PS: Don't you hate it when you run out of USB connectors on your mobo