Actually once you get a speed input for the tcu thats all you need. The tcu has a programable output for the speed output. I am using a stock 400e speedo which uses a the same input as the sl.
Yes you are correct that you need a speed sensor either the abs sensor or one on the driveshaft or wheel speed. Any configuration can be used as it is just programmed.
The problem with the CAN is that you also need to send messages back to the other control units. This is the reason you could not just stick a old style trans or a stick shift in. The rest of the computers would go into limp mode. Also mercedes loves to change this stuff from year to year and car to car as they do updates. The demand is never going to be there for PCS to do this for the mercedes. My hope is that we can get the internal speed sensors set up for proper gear recognition and the additional controls that will give.