Its kinda hard to decipher what you're trying to ask. But are you asking what should you look for and is it better to buy a complete car rather than take this free one?
Is it a rolling chasis?
When shopping for a new project you have to ask what you want to do with it. Do you want to race it? do you want to put a newer engine or do you just want to make it run and enjoy it as long as possible letting it break down and fixing it as you go? It's going to be money either way.
Mustangs are unibody as krash stated, you want to see if this dude's cars have frame rail rot on it, check the cowl to see if it leaks, floor pans, trunk quarters etc... See if the body panels are straight, look for rust bubbles. If it's got major rust in any of those areas you might want to pass if the car doesn't have sentimental value or if you're not looking for something to put a lot of time and effort and money into. ask for pictures of the underside of the car or better yet go see it.