I've looked into this as I have a really good pioneer unit that I was going to put in.
I can find the double din facia conversion and the canbus interface, you also need an ariel adapter, the central locking light and airbag off warning light as a seperate switch as they are part of the oem facia which has to go. This lot costs about 250 quid ish so not too bad....
This will help you keep the steering volume controls & the track up/down function BUT you will lose the use of the telephone buttons, the voice button and sync. Sync is no problem nor is the voice as most head units have a better solution anyway.... The biggest stumbling block for me is the loss of the parking sensors and the parking sensor graphics. Also the double din facia lays at a strange angle and points to the ceiling too much. I have given up on the idea of aftermarket now and have started improving the oem. Sub is in, as is loads of sound deadening which really improves things. Speakers and amp next, and reverse camera too.
Also... If you have iPhone set up CarPlay its so much better than the ford sync!! Go to settings- general- car play- and follow instructions on the iPhone using the voice button on the steering wheel. You can then have a short press for the ford sync or a longer press to get Siri. It even appears on the top screen as Siri. Works really well.