Is the 300D a turbo or non? If it is a turbo forget the whole thing. The reason MB never put a manual in a Turbo diesel is that 5spds they were making couldn't handle the torque and were going through clutches and throw out bearings faster than I'm sure you (average-joe-consumer) would like to replace them. If you don't like the 5spd gasser, just shoot me an email, as I would be happy to take it offyour hands!
-M-