Have fun! I rented a car and instructor through RSR Nurburg in 2014 and had a great experience. Raining, no traffic, french instructor yelling at me all the way around to keep it floored through fog and blind corners. Wife in the back seat pretending to have fun. Quite the memorable experience.
Best piece of advice I got: rent the slowest car they have. YOU will by far be the limiting factor, and a fast car only gives you more to worry about. Takes hundreds of laps to learn the track well enough to push the limits of any reasonable car, even the Twingo I drove.