perv,
good luck tossing a decent screen into that housing. been there already. size will be the issue for the earlier ones. as for setting it up with a car PC - yes. use linux and not windows. there are a LOT of linux apps that will do GPS. as for the TV idea - yes - find one with dual diversity or dual tuners. a single tuner WILL NOT do it.
I'm currently researching this idea as well. mapping the OE buttons - you can try - they are a proprietry databus. best if you are good with hardware that you pull out a DSO and trap a few signals off the Tx & Rx lines. the rest of this is easy enough.
so are you a software engineer or a Electronics engineer?
can you make the thing stable enough so it doesnt need constant rebooting?
as for getting audio and video into the OE system - if you are capable of doing what you are thinking then it shouldn't be a problem.
google is your friend on those issues. I have the AV leads here to do it.