If you are planning on running HPT, you are stuck with Windows but there are a couple decent front ends that I've seen out there... not that its a bad thing, I just personally don't like having to pay for an OS that doesn't get used often... I've been trying for the last couple months to get it to run under Ubuntu with no luck (the last thing that's keeping my laptop tethered to Microsoft), which, like yourself would like to integrate HPT into a car computer.
You might want to take a look around and see what others have done as far as boot times, since Windows isn't really the quickest to boot "out of the box"
For an in-dash, I've been looking at this one:
http://www.lilliputweb.net/ts701.html
Comes in ~$300, but has some of the "all in one" you were looking for