I want to get an old car, is that a good idea?
My first instinct was to tell you to buy a late model car since you don't appear to know very much about cars. Then I realized that 25 years ago (when I was 15) I could've been asking the same question. I bought (ironically enough) a 1966 Ford F100 p/u. It was a one-family owned, all original, rust free LWB that I paid the handsome some of $500 for. It had a 240 c.i. inline six and "three on the tree". When I bent some pushrods (yes I over-revved it) it was either learn to work on it or walk.
Well, 25 years of working on and building cars for a hobby and I am forever thankful for that old Ford. I'm a GM guy these days but that old truck is what got me started with my love of modifying vehicles.
The Mustang, like that old truck, is brutally simple and parts are readily available. On top of that, it's appreciating in value, so your efforts won't be for nothing when it comes time to sell.
So I say, buy the 'Stang. Learn to work on it yourself (it WILL break down but that's part of it) and I hope you develop the lifelong love of cars that I have. You'll meet so many great people in the car hobby. I know I have!