Check out crutchfield.com and have a look around. Any double DIN headunit will fit the same or nearly the same as your existing headunit. If you put it in and there's a tiny bit of extra space around it, you can purchase a plastic trim piece to go around it, but you're not likely to need it. Crutchfield will also list the part numbers for harnesses and so forth that you need in order to keep Sync, steering wheel controls, etc.
Here is one model I was looking at for myself. You can purchase a GPS module for it if you want to add Nav as well. If you buy from them, they will also send you detailed instructions on hooking it all up. You can save a bit by taking those part numbers over to Amazon and buying from there, though you won't get the instructions. In you buy from Amazon, you're probably looking at < $450 for the headunit and all harnesses. Add another $300 for the AVIC-U260 Nav unit if you want Navigation.
If you buy the harnesses, wiring is basically just plug and play. Connect harnesses to new headunit. Disconnect old unit from vehicle. Connect new unit to vehicle.
Getting the center of the dash out is actually surprisingly easy in the 3rd gen Expeditions. Pick up a set of plastic pry tools like
this one and make sure you have a couple screwdrivers handy. I would also suggest some masking tape and a Sharpie so you can mark the plugs that are not color coded. That's about all you will need.
Here's a video that shows how to pry out the center piece. The video is mostly spot on, but note that at the bottom of the panel it does not simply pop away like the top does. There are 2 clips holding the bottom of the panel on. Once you pry out the top 3/4 (which comes out really easy), you can peer in there and you will see the metal clips. They pop out pretty easily also, but you need to insert the pry tool and little further in and exert a little more force. Once that panel is detached, just unplug everything; make sure you mark the plugs that are not color-coded so you know where they go later. The stereo is simply screwed into a metal mounting frame. You should be able to re-use this frame for your new unit. On some of my previous vehicles, I have had to use something like
this to attach the frame to the new unit (if the hoes don't line out), but I haven't had to do that in over a decade.
Even if you decide not to dive into installing it yourself, you can take the unit and wiring harnesses down to any local stereo shop to have them installed. The Expy is really easy so don't let them take you for a ride on install costs.
