Unless changed recently, X-plan is available on current and previous model years. So left-over 2016's and current 2017's are eligible. You'll have to wait until Jan 1, 2018 to use X-plan on a 2018 model.
Dealer participation is optional, which surprises many and I have been told no by a small dealer once on a new F150. You might be hard pressed to find a dealer willing to accept X-plan on anything all-new or completely redesigned. So its 100% YMMV. Ford also periodically excludes vehicles as they see fit. Sometimes low volume, sometimes not.
I've been an EAA member for years and membership includes X-plan, and my employer has a few thousand Fords so we get the same X-plan pricing, except they call it fleet-supplier discount. Its a good way to buy a vehicle and I have used it several times. It will encourage you to not trade anything in.