Just looked at the SLS site. There is an English language button both on the home page (top left corner) and then again on each page (top right corner). However, not everything is actually available in English, including the lock/key information.
The set they sell seems to contain all items containing a lock completely: gas gap, door handles, trunk lock, even glove compartment lock and costs almost EUR 600, i.e. about $780. This is for the 280 SL as from 9/69. According to the information provided, it is possible to convert older models to the one-key system, starting with the 250 SL chassis number 2980, however then some parts of the steering column will have to be modified, up until the 280 SL 9/69 mark.
Maybe it is best to send them an email with your vehicle data and a description of what you are looking to achieve.
P.s. Ami - I am getting used to your style of English now!
Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic