I have a 84 300cd.
Go here(its a division of Pelican Parts)
Diesel Discussion - PeachParts Mercedes ShopForum
Anything and everything about these cars has already been worked out
you can also go here
http://www.benzworld.org/forums/w123-e-ce-d-cd-td/
Here is a list of How-to's
http://www.peachparts.com/Wikka/DoItYourSelf
Changed mine to a 4 speed. I enjoy it as a DD. easy to work on and of course the build quality is top notch