Asuming the battery is good (not decent but good) and the engine is in decent shape and has been properly maintained it should start in 0 temperatures without an auxiliary heater.
Operate the glow plugs for at least 60 seconds and hold the clutch disengaged while cranking. Do not expect success if the glow plugs are not functional.
Regarding engine heaters, the freeze plug heaters and tank type coolant hose heaters are most effective.
I certainly would not recommend the torpedo heater alternative without extraordinary care and observation.