LYL:

The plugs specified by Mother Benz are:
...Beru 14-7 DU
...Bosch W7DC
...Champion N9YC

All are non-resistor. It is almost inevitable that the folks at the parts store will get it wrong; they are quite likely very nice people, but their look-up information is geared to using what the plug manufacturers consider to be an appropriate replacement for 95% of the engines out there, without acknowledging the 5%. I think it would be well to change to non-resistor.