As previous respondents have noted, this is certainly one of the most hardy timing chain systems of the Benz engines, mostly due to the ratcheting tensioner.
That said, even if you did want to replace the chain, there is some sort of misunderstanding on the chain replacement price. If they will remove the lower timing cover for $300, the difference to replace the chain is insane. If they have it down that far, changing the chain would just be a few extra minutes of work, and that chain is relatively inexpensive.
You need to get a clarification on this.