I did this repair not too long ago. It is pretty straight forward you just have to plenty of patience and make sure you clean all the old gasket out or it will leak. When I removed it, the old gasket was hard as a rock.
In the 300D you have the steering box in the way. To get to the bolts that are near it I used a 6mm hex wrench attached to a vice grip for added leverage. For the rest I use my 6mm hex socket/ratchet.

Loosen the cooler lines first. I use a cresent wrench to do this and then remove the oil turbo line which I used a 19mm box/open end wrench. For the oil pressure line, I used a 12mm flare nut wrench. Once all the lines are loosened, you can remove the 5 hex bolts. The housing will be either be stuck in place or it will simply fall off. If stuck, gently pry it off the engine block. There is some plastic valve pieces in the housing that can get broken, so be careful as to not slide the housing back and forth against the block.

Again once off clean the mating surfaces very well. This is very important. Replace with new gasket dry (no silicone sealant) and reverse the install procedure. I couldn't use a torque wrench to tighten the bolts because of the space limitations, but I tightened it to what I thought was good (and I couldn't tell you how to do that... I just know).

I got my gasket from a local parts store here in my town, but others say to use only OEM Mercedes gaskets because they last longer. Not sure what to believe because my local parts store gasket seemed to have the same material as the one I took off.

Here is another thread on this subject which Brian Carlton has some good tips here.

http://www.peachparts.com/shopforum/showthread.php?t=158626&highlight=oil+housing+gasket

__________________
CJ

1983 300D 12X,XXX