The Sony Xes-Z50 is A Complete Car audio System and to answer your question it does contain a headunit, However it is not Solely a headunit and just as a number of manufacturers produce traditional headunits with digital optical outputs that need external dacs and amplifiers before they can be used they are still discribed as headunits, It is common to define the headunit as a device with a function to either control or display information about the car audio system in the same way a 7 inch pop up tft used to control a changer mechanism with no tv or tuner option installed so it does not actually reproduce any output or store any music internally but will control or direct the output or reprodution from other components of the system. And this is why most High End systems are often parts of component systems.
Information on the sony system is available here http://www.itp.net/features/980950166753523.htm and http://www.ukdigitalradio.com/products/incar/default.asp
and curent trade price in the UK is about ?000 plus fitting for the main parts.

Cheers

Max