On my 97 SL320 and C280 the OBD port is just above the throttle peddle under a flip cover. ( assuming a left hand drive car ) I'm pretty sure my 97 E320 parts car has the port in the same location and I'd expect the E300 to be in the same place. All of my cars have Bosch ME 2.0 / ME 2.1 engine computer

The in car OBD port only gets you to the engine computer, all others are accessed through the under hood 38 pin connector. There are 38 to OBD adapters however this only gets you to the engine computer.

For 98 ish + , the OBD port was used for all computers and the 38 pin under hood went away.