I agree with yoteO, walk away from the car. Most of us really care about the looks of our car. If, you can see that much rust, what can't you see. There are plenty of "good" Mustangs for sale, just do your research.
My last Mustang was bought from a dealer, something I don't normally do. I saw the car, looked at it and didn't return for three weeks. Why, was it still there, probably because it was black and maybe the tires (Slicks?) I also, noted it had a brand-new top. The second time back I took it for a drive and straight to my mechanic, after he gave it a clean bill of health I returned the car to the dealer.
At the time I was driving a 2008 white GT/CS convertible 4.6. This wasn't a Ford dealership, and it soon became apparent the salesmen, nor the manager knew anything about Mustangs. They made their offer and I left. They came out asking what I wanted. I walked away with a lower price, new tires and a full tank of gas and a free oil change. (I was trading in a car with a full tank). Additional I got for my 08 a $1,000.00 less than what I paid for it four years earlier. I'm not bragging, just saying be patient. There are good cars out there reasonably priced. The car I bought, had everything I wanted, plus.
Good luck!