Before You proceede with this keep in mind You'll be changing a file so make a backup copy before You begin. Now, You'll have to edit your player file (.PLR file) and add the following lines to the [GAMEOPTIONS] section:
The Medical and Safety cars are just two bonus cars you can drive, they do not function as medical/safety/pace cars in the game. In fact, the cars are pretty lame looking and do not handle or perform anything like the real Mercedes-Benz vehicles because they have been given the VEH file from an F1 car, they don't even have a correct cockpit view.
The 2-seater is the training car from the "school" mode. -Theme Art is just some alternative background graphics for some of the menu screens. No great shakes here, you probably won't even notice the difference.
The 3 certificate levels are graduations from the 3 training school phases (Indy, Monza, Suzuka).