speaking from experience and cold weather i agree with bob , insulate your building, metal walls have almost 0 insulating factor . it just holds the air in but the heat just leaves.
i have a neat 30 by 20 10 ft wall 12.5 ft ceiling prefab metal building with a 2 inch thick wall tin outside r 12 insulation sandwiched in between and then complete tin walls inside i would consider it the minimum for this county.
i heat it with a desiel furnace from a mobile home. this gives me enought heat to preheat the building enough to do a little painting.
try to track down a fuel oil furnace out of a old mobile home . the are tall and thin take up little space. mine uses 3/4 quart per hour running full blast and will heat any decently insulated building.