I have an intimate knowledge of this subject since I went through this with my wife's Aviator. First, you need a lot of stuff to do this. The head unit is only a small part of what's required. You need the DVD drive, a couple of modules, antenna, mirror, probably amps, and a complete body wiring harness and some accessory harnesses. The body harness was a ton of money. If Ford would have chosen to make the harnesses totally separate for cars equipped with nav, it would have been a bunch easier. Plus, you have to look at how your existing stereo powers the speakers and compare that to the way the factory nav unit does. In the Aviator, it was totally different and would have required an additional amp and wiring harnesses. Then you have to strip the car to install the harnesses and equipment. When you were all done, you would have had a ton of money and an inferior navigation system to what's out there. I just bought an AVIC-Z1 and put it in the car. It looks like it came from the factory and it is a MUCH superior unit to the Ford and includes things such as Bluetooth, NavTraffic, Voice Recognition, and is accessible to all functions (including DVD) while on the move with a very simple mod. Believe me...there is no comparison. Buy the AVIC-Z1. You'll be much happier.
By the way, I would also worry about what signals are carried on the CAN bus and how that is all integrated into the system. That could present some unique challanges and may actually require some reflashing. You are starting to get into some areas where no one has ventured and you could be out there on your own.