Do you have memory for that board? I was going to use the same board, but went with a cheaper one (celeron) to keep cost down, and because I didn't like the fact that the Atom does not prioritize instructions.
I noticed you didn't list a model number or link for the screen. Haven't decided yet? Judging by the dash kit , it looks like the Lilliput EBY701 would do well. They can be had on ebay for ~$170.
I would be sure to make an ISO backup of your HDD with all the software setup how you want so when it goes bad, you can easily restore it to a new drive. Or you could look into a laptop drive a it will last longer and stand the car environment better.
The steering wheel controls are going to be sending out infrared signals with that adapter you've picked, do you have something for the PC to receive them?
I also don't see a cd/dvd drive, are you going to leave that out, many do, but I think its a very valuable thing to include, even if hidden. If you do get one, I'd suggest a slot-loading one as they withstand vibrations better.
I look forward to seeing your progress, keep us updated.