All automobiles sold in the US beginning with the '96 model year are required by law to be OBDII compliant. That said, OBDII is indeed a standard, but there are different variations of that standard, so do your research before buying anything to see that it works with all the cars you have.

For a Benz the EASE diagnostics works pretty well for the generics. There are many things beyond basic engine information that is only accessible through the expensive systems that are definitely not a practical purchase for a DIYer.

Good luck,