Parts on American cars are moderately priced and very easy to find, what you need to worry about is how many parts you'll be buying and how often. Cars that old are starting to experience all kinds of problems that will take you by surprise, electrical wires rotting and grounding out, killing alternator after alternator. The quarter panels rust out every time you have the audacity to get the car wet(hence Rustang) The upkeep will be alot higher than a new car, but you'll learn a lot about working on cars and the payoff is huge, you are keeping a piece of history alive, and look way cooler than anyone in a civic.
If the cars already in good condition it will be a lot cheaper then newer cars. Engines, wiring and pretty much everything else on older classics is pretty straight forward and you can find any part you need easily, if you're not good with cars you'd still be able to do most labor your self. A good older engine and trans maintained can go a few hundred thousand miles without a problem.