On your 240D, one of the most important regular maintenance items is valve adjustment. Although it can be done with conventional tools, the designed tools (two bent wrenches and a spring-holder wrench) make the job a lot easier, once you learn how to use them.
Another good tool to have is a "pistol-type" vacuum pump, to check and diagnose the extensive Mercedes vacuum system.
The best "tool" you can have is a good shop manual, preferably one published by MB. JEG