Is a document typed up by my father stating that I will pay for a car loan in his name, that has been signed by myself and him and notarized, legal?

Without reviewing the written agreement and knowing additional facts analysis is difficult. A contract must, however, be based on an exchange of value, known as "consideration". If you got value by use of the car which was purchased in your father's name for credit purposes and what you signed is essentially a guaranty of payment, then it would be enforceable.