Uh... by chance have you read the owner's manual?

The only thing odd about your 420 is that you can put the shifter in first. The MB shifters of that vintage I've seen only go down to second But yes, putting the selector in second while at a full stop will engage first. It will engage first if you put enough load on the engine or sent the accelerator pedal into the kickdown switch.

www.bergwerks.com offers a first gear start kit.

Sixto
95 S420
87 300SDL