Brabus and Koenig does this kind of stuff.

It is a really big work to do this alone. I have installed a PC in my car with an LCD screen. I used an inverter directly connected to the battery. The LCD is a bit more tricky to install as it needs very high voltage when turned on. I would recommand you to check out some LCD that are specially made for 12V (so no needs to solder and add some extra electronic parts) and has a VGA and S-Video input. I have no idea how neatly you can install an LCD in a head rest... Ask Brabus or Koenig if they sell only headrests with built-in LCD.
You might have a problem with the fax machine because it is not going thru a cell phone. What about using the PC as a fax machine (the PC would have cell-phone +sattelite connection)? And have a printer to print out faxes (you will have to buy a strong and powerfull inverter...).

Hope this helps, flypside will certainly better explain this for you

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