You might try pulling the injector from cylinder #1 and looking at it.
I'm in the process of replacing my injectors. I managed to strip three of the screws, so I ordered new brackets and screws from the dealer. This is in addition to the holder, seal, o-ring and injector.
I decided to replace mine because I had no history for my 300SEL and my rough idle clears up when I have fuel injector cleaner in the tank, but returns when I don't. I had already replaced all of my breather hoses and cleaned out the idle air valve. My idle is rough when sitting in gear, but is smooth in park. Whether the engine is warm or cold doesn't matter.
I had also already replaced my plugs, wires, rotor, distributor cap, and OVP, but that wasn't because of the rough idle.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino