The car starts and idles fine and also goes into gear. However, when I hit the accelerator pedal, absolutely nothing more happens - the RPMs stay constant as if I never hit the pedal.

Is this easy to fix / look at, or do I need to take it to a take? Your help would be appreciated.

no i have not. I am pretty ignorant when it comes to working on Mercedes cars.

What do i need to do? Removed the air filter and the air filter casing? Really cannot see anything as is.

check your fuel filters

Is the dreaded ASR light on? If not could the bulb be burnt out. No throttle sounds like "limp home mode" to me.

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1996 SL320 97K miles
1996 C220 130k miles
1992 500SEL 170k miles <---sold
1986 300E 216k miles <---sold

Remove the air cleaner housing and check the throttle linkage. If your 420 is like my 380, it is likely that one or both of the nylon balls that the fore and aft shaft of the linkage rides in have disintegrated causing the shaft to no longer be captive in its slot. Thus the car starts and idles fine but will not accelerate. Cheap parts to replace. When it happened one night on my father in law's car I made a temporary fix out of coat hanger to keep the shaft captive. Mark