Dark Void said it best! Although the newer minis are more efficient, heat production is largely a result of the workload. An idling 2012 or older machine will use much less energy than a new mini that is running a typical Handbrake job.
What do you plan to do with the machine? That is a versatile computer. Are you planning to limit your options because of environmental factors?. For my systems, I prefer to provide adequate cooling wherever the computer is mounted.