A/C on way out can give a hissing sound from behind the dash which might just be low refrigerant. You can check the A/C system pressure using the Icarsoft CR Pro that was recently on group buy if you have it or know anyone who does.
I looked at loads of s/w211s before accidentally buying a w212. I saw loads of 211s with condensation and damp boot issues (if you look carefully). A W212 I looked at also had the same issue, so definitely not restricted to the w211.
As well as around front footwells as suggested, check the boot all over for leaks or any sign of moisture. Damp seat backs, damp boot tray or carpets. Anything that looks like there could be or has been mould. Take the spare wheel out. Does the wheel well look spanking and shiny (like it should do, as it's always covered up, maybe except a bit of dust), or does it look like there's water marks/been water in there at some point?
One of the massive selling points of the car I bought was the spare wheel well looked like it had just come out of the factory.
If there is any sign of water (past or present), the forums are fill of helpful advice locating and fixing. Good luck, definitely something you want to get sorted!
