yes, they should be purely resistive. I would definitely consider putting a mechanical over temperature switch in series with it bolted to the heat assembly for example so that if the fan fails or whatever, it shuts off (put a 12v light across the switch so you know if it trips). When you say "small box of a greenhouse" - are you talking about the volume of a small greenhouse or a planting box in a greenhouse? If you are looking for the planting box heater, there are heaters you can bury in the soil as well as (just saw these on ebay) "behive heater" (10w heated mat you put under the hive). The heater you show is designed to run pretty hot at 12v (for melting the stuff for a 3D printer). I would consider using two of them in series, but it depends on how much heat you are going to need to heat your project.