First I don't know anything about how car electronics work. IE the electronic ac controls. Though I am sure that there is a way to move those functions to the computer you are going to be building. Just have to be creative and know some programming and or electronics.
Second. Scrap the motherboard. You are going to need one with more PCI slots on it. You will need at the minimum of 2 slots. That if you can find a card that is a am/fm tuner and a tv tuner in one and one for the wireless network card. Thats unless you can find USB solutions to those components but I still don't like the processors that they come with. Plus after looking at the specs of the processors that via is comming out with I am not all that impressed with them. Go with AMD or Intel chips. I think you will be happier. And going with a MicroATX motherboard or larger allows you more upgrade options for new equipment as well as upgrading the processor depending on how good of a motherboard you get.
Don't know about TV tuners antennaes so I can't help you there.
I don't think you can find a large hard drive in a laptop form.
There should be a number of solutions for you when it comes to your ODBII diagnostics. Thats as long as the Vettes are ODBII which they should be. Check the ODBII forum on this board for more info on that.