being an owner of an 83 300SD i can help you out with a couple of your problems.
With reguard to that black plastic "elbow", it is called a breather tube and there is an updated one, i dont think they are more than ten dollars. However, this should not be causing your rattling. Check to see if your air cleaner mount is securely attached, there are three rubber air cleaner bracket mounts that have a tendency to break. They cost 1.60 each i beleive but the breather tube alone will not cause the car to rattle. The cruise control is more than likely the cruise amp, located under the driver's side kick pannel. I think it runs around 200$ and there is a series of test to do first to see if that really is your problem. With reguard to the power locks, the most likely reason the pump has been "take out of the loop" is because of a leaky diaphram. You will need to rent a miti-vac or some other vacuum pulling guage to seewhere your leaks are. THe diaphrams are located in each of the doors, one in the trunk, and one for the gas filler flap. On the oil leak issue, i have not known one diesel owner who can honestly say thier diesel doesn't leak oil. On the fuel issue, it is most likely a cracked fuel return line, look around the injectors and see if you see fuel dripping while it is running. THese lines turn brittle with age and will crack and leak. This is a very simply fix and a very easy DIY. Sorry but i can't help you on the tranny issue. You can find tons of information on all of these issues by using the search function. Good luck and welcome to mercedesshop!!
Ryan

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83 300SD.......sold
96 integra SE....sold
99 a4 quattro....sold
2001 IS300.......sold
2002 330i.........current.
2004 highlander limited....current.