How cold does it have to be to start poorly?

The reason I ask is that I had a similar problem with my 560SEL and like you worried that it was the fuel distributor etc but in the end it turned out to be bad injectors causing fuel dribbling and bad spray patterns. My symptoms were when hot or fully cold (overnight) good/fair starts respectively but when 1-2 hours cold very poor starting combined with a loss of power.

135K is about time for new injectors anyway if they haven't been changed already.

Hope this helps!