It depends if the seller knows how to maintain the system. Buying it and then fumbling around for updates makes it more or less worthless as a lot of these cloned setups DO NOT ACCEPT updates after 12-06. This is because cloned systems are defective in some areas and the original software can detect these problems.
Then also, Mercedes has now started to make cars which can not be coded by any diagnostic which is older than the car. This means you can only look at the car codes and not change any.
Then also you need to be sure that the multiplexer is not banned because it no longer is under contract. You can do this by lending it to a proper Mercedes garage and see if they can test it for you on a car with a latest version Star software and their own laptop. I assume this is the French original and not the Chinese copy.
As time goes on these bootleg systems will become useless. So whatever you pay you need to make sure you can bear the cost of throwing it away. So bid carefully.