A civil engineer unfortunately, so if I could build this out of asphalt or concrete I'd be laughing.
I am, however a car/computer nerd and over the years have done carputer installations in my own 32, 33 and 34 skylines (and my awesome suzuki sierra!!!) and it's usually been a case of trial and error.... and error....
I'm less razor blade and more sledge hammer if you know what I mean.
Anyway,
The plan is to replace the existing screen with a 7 inch-ish touch screen with d-sub & rca inputs
The rca would (hopefully) be used for the existing hardware and the d-sub used for the carpc
the carpc would be a mini-itx job with e6300 processor and solid state hdd with m2-atx psu
i actually agree and disagree with your thoughts on XP however, I've found that using modified versions like lastxp v20 solves a lot of the usual I NEED TO REBOOT XP problems.
The front end would be centrafuse or similar.
So it would be a case of mount the pc in the boot/under seat/glovebox etc
run power and ignition to the m2-atx
run audio and d-sub to stereo and new screen.
One other option is to use a touch screen overlay for the existing screen... food for thought....
Oh and the oem buttons... i prefer the sledge hammer approach. Basically you buy a $10 usb gamepad, rip it to bits and use the gamepad buttons under the skyline buttons. Assign keyboard values to the gamepad, setup short cut keys for the front end and bob's your uncles lover. You can get a bit more technical and use a usb joystick controller like the BU0836 but that is just throwing good money away.
I've got a similar system working brilliantly in my Sierra... To put things in perspective, after 3 days of stupid driving at moreton island the wheel bearings gave way (perhaps because of the amount of time the car spent in the air.... or perhaps the hard landings) but the carputer is still going strong.
Basically my whole point is;
Why settle for patch up jobs? Why spend thousands of dollars modifying a system that wasn't that great to start with?
Touch screens are the way to go. They're more precise to use than buttons, switch's and remote controls.
Why have a 6 stack cd player when you can have thousands of songs, movies and pictures on a HDD? While everyone except me has an IPhone, most don't realise they can tether it to a carputer or laptop to have internet on the go.
Carputers are awesome, absolutely awesome. Anything less is just... boring.