You only need about 4 liters of fluid if you are going to just drop the pan. Earlier Mercedes were equipped with torque converters that you could drain and would need the extra fluid. The pan has a reusable gasket so no need to buy anything else. Just back flush the filter with a cleaner and reinstall it too. Measure how much fluid you remove and put back in what you took out. There is a thread floating around here on how to make your own dipstick.