There's no such thing as an E420SE.. It's an E420.

No Mercedes had OBD2 capability until 1997. There is a diagnostic port near the battery. You will need to build the diagnostic tool (research w124 diagnostic tool build) and there's a lot of threads on there.

Your rough running issues could be a number of thing.

I need you to answer a question for me. Has the wiring harnesses ever been replaced (upper and lower harness). If you don't have any prior knowledge of this being replaced, I suggest you check the date on the harnesses. Anything pre 1998 harnesses are biodegradable harnesses which cause running issues.