As everyone says, INSULATE , walls. ceiling, floor, door, (Prevents heat loss and reduces the volume to be heated ) A well insulated shop will warm up faster, and use less energy to maintain the temperature.
If it runs frequently enough, pipe another radiator from the central heating, if you have it.
Next best,an electric fan heater. Convectors tend to be slow, Radiants may warm you, but not the machine so well, so rusting may still occur.
The objective is to keep the air temperature above the dew point.
Combustion heaters (:Liquid or solid fuelled ) produce water vapour, so the exhaust must be fed to the outside. Plus any leaks risk gassing you with carbon monoxide. AND they must have ventilation for the combustion air to come in.
A certain amount of ventilation is important. You exhale water vapour, you perspire, so leave a small vent at ground level. (under the Up and Over door? ) for the moist air to escape.
Howard