Hi,
As far as i know here in UK the ecu is programmed in Germany and then sent to dealer. As said above the ecu has a code which links to the other components and if they do not match then nothing works.
I was looking to but a used gearbox ecu for £190 ukl as new from dealer in £490 ukl, but after speaking to a few MB technichians who i know they all advised against it and told me that the codes cause too much hassel and is easier to buy of dealer pre programmed to you chassis number and car.

Regards