I would dump the lilliput and look at the ones supplied by linitx
Radio is still pain in the arsenal, there is a DAB unit out but can't recommend the current supplier
There are plenty of DC ATX PSU's around now.
You can go straight to an AMP from a PC, I haven't but many have.
You can use an old laptop but you will eventuall move on to a dedicated PC.
Software, Windoze is the main option, plenty of frontends out there, Linux and Mac OS's are still trying to catch up, there main hinderance is a good navigation app.
Video on headrest screens, yes and no. Personally I am now going with a 2 PC set up, one for front and one for the two headrest screens.