Sounds like quite a project you have there, I wish you good luck and hope everything goes well. I own a 1988 300E and have a carputer project soon to start, just waiting on the parts. Its no way as complex as yours, I'm not replacing my instruments or hooking into other automotive systems. My setup will be a 7" LCD touchscreen and PC running Windows XP and front end software to control multimedia, navigation, mobile phone, etc.
Building and mounting the PC is a small challenge. My main challenge is to mount the LCD screen without drilling or cutting into the dash or centre console. As you would know, there isn't a double din space in the stock dash or centre console and I don't want to mod anything just in case I want to sell the car as I will be keeping the carputer if I do that. To that end I'm looking at a Ram windscreen suction mount setup as the solution, unless I come up with a better idea.
I look forward to further posts on your project.
All the best,