Problem is that my understanding is the SKREEM includes a module that is interrogated by the ECM and has to return an encoded response (why each needs to be matched to a VIN). This is why I suspect it would be easier to reprogram the ECU. Was there any -320 Merc that did not have a SKREEM ?

Let's just say during my long career, several times I was given a box, told where it went, and had to figure out what it did. This is much simpler but if had to start from ground zero would have to either make or acquire a test bed to capture signals. But even before that would need to identify the internal components. Am sure schematics exist but right now am not sure is the ECU is a Bosch box or Siemans..

Just as a SWAG I'd target a $25k budget for the project (lots less than my retired-from employer would bid).