Yes I agree, get a fuel cap gasket first. However I think you have to make sure you get the right gasket for the right fuel cap.
They have a new style cap now to differentiate it from the oil filler cap. Mine was leaking in the same way and I just bought the new style cap and it's been good ever since.
since you just got the car.
do the following:
Adjust the valves and while the valve cover is off the engine check the timing chain stretch - a good independent should know how to do this. I had the MB dealer do it for me. They also replaced the valve cover gasket (should be done)
Change the oil and filter, brake fluid, AutoTransFluid and filter, air filter, and the two fuel filters.
Run a can of Diesel Purge (it's Lubro Moly's Diesel Purge) at full strength. I got mine from Performance products and it came with a nice set of instructions and tubes to do this.
Check your brake lines for corrosion (probably not a big deal in CA), check the condition of the pads and rotors (if the pedal pulses the rotors are bad)
That's all I can think of right now!
Enjoy the TRUE coupe