I'm the go to MB guy when someone who knows me has a problem.
Anyway I think the best course of action with this car would be just to drive down and look at it in person. Then we can take it on a *test* drive to either an Autozone or a MB dealer whatever is closer. The price on the car would be great if the CE fix is $500 or less. If it costs $1k or more to shut the light off then ehh the price isn't that great. The GM dealer probably isn't going to be interested in going to nuts with it. Sellers seem to get scared off when you know to much about these cars. They rather sell to easy pray with MB stars in there eyes.
I think I will tell him to email the dealer and tell them to plug an OBD-2 into it. I can't imagin a GM dealer doesn't have one laying around somewhere.