For warranty work, any Ford Dealership is required to do the work on your car.
Naturally a dealership you bought the car from would treat you with better care, but not much better care.
The only thing required with less than 30k miles on a car is oil and filter, so I would not worry about free services on a Mustang. Cars with free services for major services are much more expensive than a Mustang.
If you want discount coupons sent to you in the mail from a dealership, all you have to do is call them and have your name put into their customer database. Believe me, you'll begin soon to get mail from them.
I did get coupons for free oil and filters from a dealership I did buy a car from...and they had no expiration date...rare these days. But would it be worth losing money on a car purchase just to get those coupons, I don't think so.
There are buyers who are repeat customers for years with the same dealership and they may get a repeat deal on a new car purchase. Unless you are thinking about that, making a committment to stick with one dealership for life, I would not worry about just where you buy a car
Looking back on what I did while my car was under warranty, I wish I'd done one more thing...I should have kept going back to the dealership when they'd tried but failed to fix some items...or when they said, "no problem found" or "unable to verify complaint". Doesn't matter what dealership works on your car, it is the luck of the draw as to if a particular problem gets fixed properly the lst time you go to them with a complaint about that problem. Don't procrastinate. If you get the car back from service and it is not right, then go back until it is right.
For service I would check with all dealers near where you live. Pick the dealer which does the most service work. The dealership which has the most techs working. That dealership is most likely to have found new problems and fixes for those on a new model before other dealers with fewer techs and fewer customers. The dealership with the most technicians working is usually BUT NOT ALWAYS the one with the most new cars sold in your area. Maybe the best way to find out would be to call all dealerships in your area and ask specifically how many techs they have.
Obviously if you get really poor service more than once from a dealership, you just don't go back. Believe me, there is one dealership I left yelling and screaming from (after they serviced my car under warranty)...and I'm not sorry that I did that.