Yours owners manual contains operational information.
The CD-Rom's are digitized versions of paperback repair manuals. Mercedes produced a "Maintenance Manual" for cars from 1981-1993. It contains instructions for tasks such as fluid/filter replacement, etc. This manual is included within the CD-Rom for 126 cars. Don't know about 124s.
If you plan on doing repairs/maintenance, you're going to need documentation. The CD-Rom is loaded with information.
Some complain about ease-of-use, or the lack of it. It takes awhile to get used to, but it's all there with the exception of transmission repair.
I believe this site sells the MB CD-Rom's.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.