As had been mentioned, there are like 20 different variables in the engine idle - could be any permutation of those variable (not necessarily 20, but its a lot).

In any case, some things to look at would be:

wires

cap

rotor

ignition coil (only way to test it is to replace it with a new one)

engine mixture (could be lean - also connected with carbon buildup on the valves that would lean the mixture out on you)

vacuum leaks (there are vacuum lines literally all over the car, any one of them could be the issue - not just the big ones under the hood, but also the little hard plastic lines too - your MB dealer should have the proper lines to replace them all if you want. don't forget that the door locks operate on vacuum too)

fuel accumulator

fuel delivery (filter or pumps?)

fuel distributor

injectors (these are probably bad - or at least the seals are which could cause air to sneak in the engine that way. I believe there are two seals - one inside and one outside - from what I understand you test by listening to engine idle while spraying carb cleaner on them - there are many posts on this already)

temp sensor (last sensor on the engine closest to the firewall)

crank position sensor

as for the MAF, it is locate under the air box and sits just below the fuel distributor - is the gold plate that gets pushed down.

The idle air valve is the cylinder that is located infront of the MAF (towards the radiator) and has two black hard rubber hoses going into it (the size of the hose that connects to the top of the air box).

Just for a reference, there are a lot of posts regarding the idle of the car that I originally posted (had this problem myself) - the killer part seemed to be the ignition coil. However, even that didn't fix the idle (still dips at stop in D and R). Speaking with my local mechanic, this is an issue that has plagued the cars since they were brand new (when he worked for the local MB dealer in their shop).

Good luck fixing it man.

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87 300E
202k on the clock