I would maybe try to put the lcd in that change thing (cassette holder?) underneath the radio mount maybe... i cant really see of anywhere else that would house a 7" lcd. As far as the motherboard goes, normally you dont build the computer inside the car, using the car as the computers case, you just build a computer like you normally would inside of its own case and just toss the pc itself in the trunk or somewhere. I would do some reading, theres a ton of info on this site. Last month was the first time i even knew that carputers as a hobby existed and today i have a fully operational laptop drivin', home built startup shutdown controller, gps enabled, custom fabricated console mounted 7" touchscreen LCD, carputer and i learned everything from just reading what was already posted somewhere.
Now all i need is a good controller!