In my opinion no we don't need any of these codes unless totally necessary.

Currently we apply [W121.010] for the 190 saloon and [W121.110] for the diesel version, so it would appear odd if we were to swap the code for a simple solitary [W121] for the roadster and hardtop.

To me generic codes are better than too-specific ones, as I already said for other vehicles.
same for Mercedes, people will search for W121 if they want that model, and then may miss all these specific ones. If one wants just a body style, there is a filter for that. And I'm not sure that having a way to filter on engine type is useful.
A solution would be two codes : a "generation" code for the generic code and then use the chassis field for the specific ones. But I don't know if it is good to add even more info or if it makes more difficult to maintain.

All Mercedes-Benz Metris are assembled in the USA according to Wikipedia:

https://www.imcdb.org/vehicles.php?resultsStyle=asImages&yearFrom=&yearTo=&makeMatch=- 2&make=&modelMatch=1&model=Metris&modelInclModel=on&modelInclChassis=on&mk=&origin=- &madein=&madefor=&role=