When I built my carpc I just used a M3 ATX powersupply like this
M3-ATX, intelligent ATX automotive power supply, 12/24V - power your PC in a car and found it worked brilliantly. The computer had no issues with being on while I turned the engine on. There are a bunch of other ones that would be better for your specific system. I didn't bother with using a netbook, as I figured I could make a faster mini ITX based system for cheaper.
As for noise you are going to have a lot of fun with that. I assume you have done the normal shortest and best earthing possible, run power wires as far from signal wires as possible etc? Does the pitch of the noise increase as the rpm goes up? I suffered that for ages, in the end it was because I ran signal wire within 1m or so of the alternator. It was normal conversation level whine, horrific.
I toyed around with a few filtering style things and they were all totally useless, not to say they are in all circumstances but this looks like a lot of money invested in something which may give you no noticeable improvement.