Using the D3 as the front & backend of a carputer really limits the true functionality of it. It'd be tough for me to do it knowing I already have a good HU, but you'll need to get a 7" touch screen to really use the system.
Do you have external amp(s) already powering your speakers? If so, you're ahead of the game.
Let me know what you want out of your system. And the most important question...what's your budget?
I work in IT, so this sorta thing is right down my alley. However, I spec'd one out if I were to build it and it would cost about $1350 to have it do what I want. That's not including an amp. The reason I'm holding off is I can't find an amp that has an optical input on it anywhere.