People that fix controllers call them "rebuilt". There is one really good source for "rebuilt" MB controllers. That place is Beckman Technologies in Durham NC.
Many of MB's controller fixes are just software modifications, because of this rebuilders can make a very good product. Unlike BMW or the rest of the world, MB makes one buy a new controller (up to 2002 when they started "flash" programming controllers) to get new software.
There is a very important issue that is not technical in your question. THe standard diagnostics of automotive controllers does not allow for absolute judgement on these issues. A tech working on a problem will often get to the point of "substitutting know good controller" to confirm diagnossis. Anyone paying for such diagnosis and then finding it not to be so needs to proceed with diagnostics. For those only dealing with this as a consumer it becomes hard to understand.
I do diagnostics for other shops almost daily. In many cases I have the "know good controller" and can finish the diagnosis. In other cases I call the shop and give them my diagnosis at that point. They have two options. The initially cheapest option would be to buy their own controller and finish the fix. The second option is for them to pay me for the fix. Seams easy right. Not quite, if I buy the controller and find it not to be the problem I proceed with diagnostics to the final fix and they pay whatever it took but not the controller. If they decide to do the fix themselves then they own a controller.
The point here is that at the point you are at, you are only paying for testing and a concept that appears next in testing. if you wish to proceed on yout own then pay your bill and take the risk. There is no warrantee on testing. If you accept testing as diagnosis and let them finish they should warrantee everything including their diagnopsis.
Bosch Master, ASE Master, L1
33 years MB technician