Single stage urethane is a 2K urethane paint, meaning it uses an activator or hardener. It's called single stage because it has the gloss built into the paint as a single step application. So it doesn't need a clear coat, though clear coat can be used over it for a deeper gloss. Base/Clear will work okay under the car also. Many use the same base/clear the body gets. Single stage will somewhat be less work since it saves the clear coat step.