^^^ Good advice. I wish I had done that first, when my 240D ignition started acting up. My cylinder was probably "ok". Being new to this "hobby", at the time, I just assumed (always bad) anything from the dealer was gonna be big $$$ and took another path.
I would only add, if his ignition tumbler is working fine, why replace it? If the blanks are available at MB, I'd just order the new key. FWIW, my E300 master key is cut on both sides (into the middle of the metal blank, can't be done on a "normal" machine). It cost $46 a couple months ago. Only my $.02.