If you dont know anything about cars stay away from the rx7, you will run out of money quick. As for the 5.0 mustang, you probally would be happy with one of those. you can even get a v6 mustang, granted its not the 5.0 but it will fit your requirements.
Can you give more details on what you are going to be using the car for? besides just waht you want? You looking for a car you can make a project out of? you looking for a sports car, a eco car?
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Driving definitions: Understeer is when you hit the wall with the front of the car and Oversteer is when you hit the wall with the rear of the car. Horsepower is how fast you hit the wall and Torque is how far you take the wall with you.