There are so many variables when buying a mustang and choosing one that I would suggest checking out this website to se what style you like first.
Then you must decide on Coupe, 2+2 (Fastback), or convertable.
The coupe will be cheaper. It tends to be less desirable than the other body styles. Do you want a V8 or In-line 6? Manual or Automatic?
The V8 289/302 will raise the price as well as having a 4 Speed manual transmission.
Make sure that what ever style you decide on that it's rust free. Rust repair can get expensive. Use a magnet to go along the body of a prospective vehicle to make sure there is no body filler. Get the VIN#(Vehicle Identification Number) and make sure that the options on the VIN match the car. Beware of the term "Rare". Everyone thinks that because they have an old mustang that it for some reason "Rare". Best of luck in your search!