I doubt very seriously that you can upgrade radios in the future because of wiring. But, check Ford's aftermarket prices. You wouldn't want to anyway. I just got my wife a 2007 Mercury Mariner. It has a single CD. We traded in a 2005 Escape limited which had the audophile sound system, 6 disc changer, satellite radio, etc. My dealer agreed to swap the radios. Fine. We signed the contract. I made an appointment for the swap. After an hour, the service guy brings me the Escape radio and a bunch of wires plus the Mariner radio and shows me how they do not interchange and explains that changing the wiring isn't possible My salesman tells me he will give me a 6-disc radio when he gets one on the lot that will fit. We are going to take all vehicles that arrive apart and check????? Sure, we are. I checked Ford's prices for a new 6-disc changer that would work (non audophile.) I was promised dealer cost. That is $500. Yes, you read right, $500. I am putting in an aftermarket. Better sound, more features, less money.
Check Ford's website for exact make up of the different levels.
And get what you want from the factory. Adding afterwards is often difficult, impossible or cost prohibitive.
My wife also wanted a message center for her Mariner. The factory price for the option which also includes lighted vanity mirrors and automatic head lights is less than $500. It can't even be added because the wiring is not there.
Adding later on often isn't realistic.