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.
You'll need subwoofers and a subwoofer amp if you have the base sound system now.
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. You can use the face plate (with some modifications) from the headunit you bought with whatever aftermarket system you pick.