I'm guessing that dealers stocked more 430's because buyers ( leasers ) tended to want fully optioned cars and the 320 was more of a fill in budget / teaser lease car.

You could use the fewer 320's as an advantage when buying used. In the land of R129 , the 6 cyl cars are somewhat unloved and prices are lower. However, the 6 is just fine for general use and does not run out of breath on long mountain roads. Plus is makes nice sounds when leaned on.

I don't know for sure if the CLK 320 has enough power or not but I'd expect it to be fine for your needs.

I " think " both the 320 and 430 have issues with EGR or AIR pump passages clogging in the cylinder head tripping the check engine light ( and failing emission tests )