I am 100% sure you could get more then $21k for it if sold privately. I would be interested if it was an auto. Not a fan of the Chinese made MT82 and I love rowing through gears.Right now its got about 19.5k miles and gets bet. 18-22mpg.
Only 1 car? That is a bummer. Get a cheap good gas mileage DD that will see bad weather. this will keep your new Mustang nice and clean.Yeah I probably could, but since I live alone it would be very difficult timing it right and getting around if I were to sell it until I get a new one.

haha, that's never going to happen for me...only got 1 income, house payment, student loans until I die (almost) and car payment. Live paycheck to paycheck and don't even have health insurance or retirement at the moment.Wait until you can afford to purchase the car outright.
In my opinion its better to avoid finance charges.
There will always be something shinier/better available.
My 16' auto GT with 3.55 gears in sport mode at wot feels faster (sotp/buttdyno) than any of my past mustang GT's did, stock.I finally checked out a 2016 GT at the local dealer. It was a base auto, ruby red with over the top stripes. It was pretty nice. Auto shifted good, but did seem a tad slower than my manual with 3.73, but couldn't really get on it much since the dealer went with me and didn't mess with the sport mode.
That dealership also has an orange base auto with 19" wheels and 3.55 for 30k which sounds like a pretty good deal too, just wish I could know if they would give me the same ballpark for my trade and such.
This.Sounds like you have to do a lot of financial gymnastics to afford this car.
If you are living paycheck to paycheck, with no health insurance, student loan debt, and I'm assuming no cash emergency fund, then this is the wrong car at the wrong time.
No sense in a having a great car with no money for gas.
Keep the '14 GT and buy an new S650.
Just my 2 cents.
All the best.