How's your clutch control?
If your passengers can feel your gearchanges, that is probably because you're not changing gear very smoothly.
I have been a passenger in a W202 C180 manual and in many manual W124 taxis. The gear lever does seem to "click" or "thunk" from selection to selection. Is this what you mean?
I always found it a satisfying reminder of how well built a Mercedes is: one of the taxis showed 457,000 miles and the driver told me that was all on the same engine and gearbox (2 clutches, though).
Why do you want a manual anyway? My advice is: spec your car with an automatic transmission. I don't see why you shouldn't pay for it, although you have some leverage in the fact that your car is being delivered late because it was damaged in transit. It will depreciate less. The foot operated parking brake will not be a nuisance. It will be smoother. It will be better. Unless you are a racing driver, you will not be able to drive the manual car faster.
1999 SL 500
1993 320 TE
1986 500 SEC