Does the work shop have walls, a roof, and closable windows and doors? (is the interior protected from the elements?)

I flower in a 2.5m x 3.5m outdoor shed, (without additional cabinet) and temps are getting down in the 30's overnight, so am forced to use a heater on the next to lowest setting.
A little trick I discovered last year (sometimes it takes me a while) was to have a dedicated fan near the ceiling above the grow, that forces the warm air back down, directly onto the crop. Works great, and not nearly as expensive to keep warm, as without the fan recirculating the heat that has risen.

Setting up your growroom to 'dry-run' a few nights of actual growing conditions could help steer you in the right direction, since we dont know how big or drafty the shed is, or how cold y'all are getting at night...