My 77 240D had no issue starting up in the midst of the coldest Chicago winter, usually after one glow cycle. Never used a block heater, I did treat the fuel during the winter with Power Service Cetane Boost (
Robot Check )
That made a noticeable difference in cold running smoothness. I would also always turn the idle adjustment knob to the furtherest most left (fastest idle) when it was that cold. I would run it really fast for a few minutes then adjust it back down to a normal idle. That seemed to help in very cold temp starts.