I lived in Minneapolis for four years with my 84 300SD and started it at temps as low as -10F, roughly...thermo read -23.5 C.

New glow plugs and new battery, no block heater hooked up.

Preglow three times then crank it for about a good 45 seconds.

I routinely started it at 0 F without problems. I think that Bosch plugs and a fresh battery are the key.