I just did this on the E320 last night. The chain on it has colored links that line up with the cam sprocket marks when the crank is at TDC. In addition I never let the chain get "out of sync" with the crank sprocket so I never lost that relationship. Cap'n I would think that you should be able to find some marks on the cam shaft that line up when the crank is at TDC.
Working on this engine makes me want to purchase another W123