Whenever I have purchased a Mercedes filter that's an insert type, it's always included both the o-ring and the drain plug washer.

Buying the parts from another source may not include these items but I suspect that if it's an OEM replacement from Germany that it will.

However, even if the o-ring and washer are missing you could probably use the old ones until the next change. I had a whole toolbox full of these items that accumulated after years of servicing my wife's 190D and even though they were used, they were perfectly OK.