The spark plug issue is resistor versus non resistor rather than electrode design. The only readily available line of non-resistor plugs is Bosch Super, and the H9DC is the proper plug for M103 engines. They're easiest to find at Mercedes parts specialists for about two bucks each. Most commonly available platinum plugs are resistor type and the non-resistor Bosch Supers are not commonly available at "McParts" chain stores. You can try, but be sure you don't buy HR9DC plugs. These are resistor type Supers - note the "R" in the plug designation.

Resistor plugs add more resistance to the existing RFI resistors in the cap terminals, rotor, and plug end terminal on the wires, which alters the wave form and spark energy and is a probable cause of rough idle and high HC emissions due to misfires.

The OE cap and rotor are high quality - expensive and should last at least 60K miles, but you would be wise to remove them for inspection. You can dress the brass terminals with a sharp steel dental scrapper or Xacto knife to remove oxidation. DO NOT USE SAND PAPER.

At 60K when I inspected my cap and rotor I found a "flaky" resistance reading on one of the terminals. The RFI resistor should read about 1-2K ohm IIRC. One of my cap terminals had high resistance but dropped if I put some force on the tower.

I replaced both the cap and rotor with Bosch replacements - equivalent to OE, and idle quality and emissions test performance improved.

You should check the plug wires and inspect the spark plug connectors for damage. The wires should all measure about 2K, most of which is in the RFI resistor at the plug end. Many are damaged by ham fisted mechanics who pull the wires from the plugs without using a proper set of spark plug wire pliers.

Duke