To add factory navigation, you need...
The head unit.
The DVD navigation unit (mounts in trunk, head unit is dead without it).
Map DVD (about $200 for a recent one).
A different rear view mirror (the voice command microphone is built into the mirror).
A four channel amplifier for the door speakers (the nav headunit has no amplifier, unlike the non-nav headunits).
New speakers for the doors, the nav speakers are slightly different.
The navigation steering wheel (the radio buttons are different for the nav option).
A whole bunch of wiring harnesses to make it work. (Note that even though the connectors going to your non-nav unit will plug into the nav headunit, the pinout is mostly different, and won't work at all.)
It'll be much much less work to install aftermarket instead.