Ah, Minnesota! Grew up in south central MN, Now in Idaho. I would suggest a ceiling unit with divorced combustion or whatever it's called. Anyway, it uses outside air for the burners which is drawn in by a fan and then the exhaust is blown out so inside air does not reach the flame. I recently bought a Modine 45,000 BTU unit last winter but haven't installed it yet. It came with both propane and natural gas setups. I found the best deal from a greenhouse supplier called ACF Greenhouses. Their website is
www.LittleGreenhouse.com
I was using a dual burner tank top infrared propane heater. It did a so-so job but I didn't think about the moisture these things produce. Condensation settled on anything that some mass to it like big sockets, ratchets, drills, saws, etc. This condensation seems to have a corrosive effect, too. A lot of stuff rusted in just a few months. Thus the furnace with outside combustion air!
Since your walls and ceiling are still open, now is the time to insulate and sheet rock! Insulation will pay for itself in saved fuel and comfort. If you have plain metal doors, insulate them too. That is a lot of surface area for heat to escape when it's 30 deg. below with a 30 MPH wind!
Another idea: When you sheet rock the ceiling, add an attic ladder and put down some plywood for a floor and store parts up there. Keep the heavy stuff near the walls and sheet metal type stuff nearer the middle.