The 300 is easy, the e320 not quite.
The problem with the e320 is that many systems have onboard diagnostics and the information isn't sold by model and year its sold more by system. All of the data, theory, and diagnostic procedures do not exist in one location (unless it be my office [bg]). Proper diagnostics would require at least two forms of manual: the ETM (electrical troubleshooting manual) and the DM (diagnostic manual). The ETM can be bought with the 124 CD. The DM is a professional grade manual and if still available in print would cost over a grand I would guess. The DM is purchasable by the system; such as A/C. I have 16 volumes, about five feet long on a shelf. These only go to 98 as after that its on the MB info system WIS.
The tool necessary depends on the level of access needed. Full access would require a SDS which runs somewhere over $21k. The best aftermarket tool I think is the Baum tool www.baumtools.com
, which I see is available right now for 2000 plus modules. A very good tool for a little less money would be the AST tool www.asttool.com
The car is too old for the cheap PDA OBDII tools. They only do the engine side of powertrain anyway.
Your car can be accessed by the impulse method for most systems for fault codes. To try and repair a car from fault codes alone would be similar to interpreting a novel from reading the table of contents. Ninety percent of diagnostics is done with actual values, activations, and adaptations, only available in the higher scanners.