Having run diesel engines in some really really cold conditions, I found (OK, my employer found, I went with what works) what's now CRC Fuel Therapy to be the best diesel treatment. Takes care of gelling, condensation in the fuel and injector cleaning.
During a near Arctic condition, I took the battery inside to keep it warm. Dropped it in place in the AM and the car started right up. In a related item, military vehicle specs for the European far north/mountain regions had (once upon a time) the batteries not only in what amounts to the interior of the vehicle but they had heating pads. I guess the draw for the heaters was less than the drop in efficiency from cold. A hundred watt light bulb in the engine compartment and/or a block heater helps. Not sure if a LED bulb puts out the same heat.