Hi, at this mileage, around 50000 I think the likely hood of plugs being the issue isn't as great as other things. It is cylinder specific so that makes it easier. We can do a lot of guessing but you can also take it to a dealership that has the MB diagnostic system. Plug wires are a big issue with that engine group. There is a test that allows a technician to shut of one half of the ignition system and watch the other half function. As you mentioned there are two plugs per cylinder and of course two wires. The wires were around 60.00 per so it gets costly. If there is a secondary insulation breakdown it will cause misfire at high ignition loads. The mileage is a bit low for this problem but it's worth checking as it can get quite expensive quickly just in parts. At 80000 miles when they started with a misfire on one or two cylinders we recommended replacement of all wires or the car just kept returning with another failure starting in a cylinder not yet addressed. The coils can also be an issue, I have changed a few but no where near as many as plug wires. If you look in a tool manual you should be able to locate a valve adjusting wrench for an older 110 motor, it seems to be the universal plug wire removal tool. It hooks under the end on the insulator and levers the wire up and off. The two rear cylinders on this engine are just a bear to got off sometimes.
Bill