The Xenarc model you reference has VGA input built in.
The touchscreen is USB.
It is specifically made for PC use.
So you wouldn't need a VGA-RGB conversion device and you could use the touchscreen with the PC if you go with the Xenarc model.
As far as the units by Kenwood, Alpine, etc., you are correct. You won't get touch control of the PC and you'd need to get your video to a composite signal. And the video quality will suffer, as PC video is a much higher resolution than simple television.
As far as developing an adapter, I have no idea.