Ford Mustang GT Coupe
Overall: I personally wouldn't recommend purchasing this car, however Mustang shouldn't be considered a total piece of junk. You could easily win a few races in this car. It's also very easy to handle. Nevertheless, there are more interesting (and faster!) cars in the E class.
Ford Mustang GT Convertible
Overall: I guess you may be surprised by the fact that the Cabriolet version isn't slower than Ford Mustang Coupe. As a result, you could consider paying 5000 dollars more for this car. You should also remember that you will be allowed to upgrade it at Saleen!
Overall: This is a very nice proposition, so you shouldn't ignore it without looking into the details. Ford GT has an excellent acceleration value. It's also very stable at high speeds. You shouldn't complain about the maximum speed (~340 km/h), however you will find faster cars in this group.
Ford Shelby GR-1 Concept
Price: You will receive it automatically once you've won all SPEED challenges.
Overall: I wouldn't say that it's an excellent car, especially since you won't be able to buy it the usual way. GR-1 has a very decent maximum speed value, however going through corners is not something I would call enjoyable. It's best to stick with the Ford GT.
Ford Shelby Cobra Concept
Price: You will receive it automatically once you've won all MODELS challenges.
Overall: Cobra Concept is much slower than GR-1 Concept, especially when it comes to the maximum speed value. Most of the B class vehicles can go faster. Additionally, you can't forget that you won't be allowed to upgrade any of the concept vehicles.
Ford Mustang GT-R Concept
Price: You will receive it automatically once you've won all CAR DELIVERY challenges.
Overall: This is a very interesting vehicle. You should know that GT-R has nothing in common with a standard Mustang version. This car is very fast, especially if you remember that it's the D class we're talking about. Additionally, it's very easy to drive. A truly great piece of machinery!