John, my '79 300D won't even fire on a cool summer morning if I don't use the glow plugs for 3 or 4 sec. In winter, (-5F) I glow for 12 sec. After cranking for 3 sec, it starts right up. The good part is that the plugs stay on while cranking. On cold days you only get one chance at it. Glow plugs are much easier and cheaper to deal with than a worn-out starter.
I am not sure about the batter size, but it sure is heavy!