Can the MB nav unit be purchased and installed installed after vehicle purchase? How about another brand?

The COMAND can be installed after you buy the car. The dealer will tell you it can't be done, but they are wrong. Most of us who have had this done purchase the unit from Steve Krabbenborg. Here is is COMAND web site: http://www.mbenznl.com/mods/comand.shtml

Aftermarket units can be used also. There are some drawbacks to doing so including cost, the steering wheel buttons don't work, they aren't compatible with the existing stereo, and non-factory appearence. On the other hand, the aftermarket units are DVD based and have many more features than COMAND.

There are aftermarket add-ons from Garmin and Magellan that have no moving parts! I use a Garmin iQue 3600 and, aside from the additional wires that I choose not to hide (too much effort for a leased vehicle), it does everything for me that I need. Besides, at the end of lease, it transfers to the next car and I don't have to learn another interface!