I have given this alot of thought. Your first power option is the best.
Why even have a headunit? The PC can serve all the functions you'd use the headunit for and with a remote or hotkeys. You could serve all the same functions, add a PC TV/FM card, XM PC reciever, High Quality sound card and use a line level or digital output to an external amp powering the speakers.
The screen could be slide mounted in a Pre 98 Console, along with the DVD drive. Use a USB 2.0 enclosure electronics to run the Drive and make it slot load. Stow the system under the passenger seat or the trunk in a smaller enclosure.
