There has been some write ups on these and if I can find it I will post it. Basically there is some evidence that it is better not to let a diesel get that cold.. Frankly I don't see it as a problem as long as you are not starting it when it is really cold (thermal expansion rate issues) There will be some stresses on the block when it gets cold though, no question. Deisels just don't like being started real cold. Lots of times you will see more smoke in the late fall and that is due to the current engine and fuel temp.
I think the so called "bilge heaters" are expensive because they are "marine" and will work for gasoline engines and are spark arrested and can be exposed to moisture.
My uncle use to keep his big boat in the water year round in a canal behind his house. The water would freeze up thick but he had a "bubbler" installed on the bottom under the boat and it ran all winter to keep the ice clear next to his boat. He also had a bilge heater to keep the bilge around 40 degrees. He would start the engine thru-out the winter from time to time.
I have two of the cheap heaters in my boat now. (they are not in the bilge but the thought has crossed my mind to get a real bilge heater as well) One is not much of a heater but more of something to help combat moisture...moving slightly warm air in confided places . The other is a cabin heater...sort of like a toaster with a fan. I use it when I am on board working on a boat project. It can warm up the boat to whatever you want...it is nice in there.
Neither are made for wet environments but if your bilge is dry and you are out of the water I suppose you could put one of those 79 dollar heaters in there (but it is intended for cabin space)...there is a freeze setting on the thermostat to not let the room go below 38 degrees. I only turn these on when I am on the boat during the winter.