On an 84 month loan, you will be upside down for at least 5 years on the Mustang.
As a stockbroker / financial adviser for the past 30 years, my advice to my clients is that if you cannot afford the payment for a car on a 36 month loan, you cannot really afford to buy that car (and should really consider something cheaper).
Mike, you obviously know your stuff. This is good advice. Now personally I didn't follow it, but I absolutely agree in principle. I have a loan on my vehicle, but the car is worth more than I owe because I put a good chunk down. At any point, and for whatever reason (if I see fit), I can trade in my car and be in an equity position. I figured my cash was better spent investing when I can earn more than I am paying in interest. And I am a car guy and had to have it.
And on that note, OP, I have a small suggestion/question. Can you wait a year or two? If you waited for, say, the 2017 Mustang, you won't be upside down on the Focus (I would imagine). Ford will up the horsepower of the car by then (again, I imagine), and you'll have the latest/greatest Mustang with a good three years where it'll be the current generation body style. In addition, it gives Ford time to work out the kinks. Finally, you will get a MUCH better deal as it won't be the hottest new car release.