I'm trying to do pretty much the same thing. This is the way I'm leaning. I decided on using a netbook since I just don't need a lot of computing power. The netbook came with Windows 7 installed, which has some quite nifty power management options.
Now I gotta read up on USB hubs, com ports and such to know what I can get away with since with the netbook, I'm really a USB input or two short. But there again, I think an MP3C DC USB hub would fit the bill nicely.
Just start reading and "window shopping" around the various dedicated CarPC retailers...there aren't that many...and honestly, everything you need can be found at either of the two "big" car PC sites. One of them being this one.