Quick answer
Buy in this order: automatic vent openers (overheating kills more plants than frost), then shelving to use the height, then a heater only if you grow through freezing nights. Size the heater to your greenhouse's surface area, not its floor area.
GearGreenhouse heatersElectric and propane greenhouse heaters, sized with the BTU formula, plus thermostats, running costs and safety.
GearGreenhouse shelvingGreenhouse shelves, staging and benches that actually fit 6-ft and 8-ft kits, with layouts, load ratings and materials.
GearGreenhouse fans and ventsExhaust fans, automatic vent openers and shade cloth, sized with the 12 CFM per sq ft rule for hobby greenhouses.
Related guides
Heating and ventilation work together. Our winter greenhouse guide covers insulation and thermal mass, which can halve what you spend on heat.