Heater recommendations for greenhouse during winter months?

[big rockpile]

I have 400 Sq. Ft. And they suggested this Heater.

I will put it half way up on one side. Set it down low and have two big Fans up high to circulate the air.

Wanting to keep it about 55F.

Snow normally we get very little snow but we need some.

big rockpile

 

[big rockpile]

I'm switching to Propane, electric really cost and if the electric goes out when it's cold we have a problem.

big rockpile