First up, welcome to the group!
Thats looking to be a very nice setup, retro meets technology, looking forward to seeing it progress.
If you want to control things like power windows, it might be worth looking into a fusion brain for that, as you could probably also interface it into the airbag suspension, I believe someone else on the forums was having a go on airbag control at one stage?
For the computer, I think this is the kind of situation that is calling for a fairly beefy system, rather than the lean atom computer setups. I would recommend a mini-itx board with a lower power draw i5 processor (Not sure if the second gen Core i series is available in a mini-itx package, as they use a new socket type over the first gen).
But you would want to have speedy startup, but also have a system that is responsive, you dont want to be waiting if all you want to do is put a window down to yell at some git who cut you off in traffic
Add in an SSD for the primary hard drive, which will speed up boot times, and you should be all good.
I'm not sure on the dimensions of your dash, but can you fit a 10 inch or larger screen into that dash area? I think a minimal, retro dash would look awesome with a nice BIG lcd panel in the middle