That is MB's online Workshop Information System. It consists of all of the factory maintenance procedures for all models. I don't have a subscription but I do have a DVD for all W210 models which was for sale up until about a year ago. Unfortunately, I don't believe there is one for the W126 either.

FYI - There's a thread on Front End Rebuilding over on:

http://www.benzworld.org/forums/w126-s-se-sec-sel-sd/