As you noticed, asking this kind of question gets as many answers as questions like how high is up or how long is a roll of string. DocHeb was RIGHT ON THE MARK when he suggested radiant heat. Aye Cap'n I cannow change the laws of physics, the shields won't hold and we'll lose containment. I truly love in floor hydronic heat but it isn't fast and requires that you heat the space to be used basically all the time in heating season. In floor hydronic is a lot like a heated swimming pool, extremely comfortable but not something you can do on the spur of the moment when you need to fix something. That might be just the ticket if you are going to spend a lot of time laying on the floor and don't mind the expense of constant heating. Infloor hydronic might save a few percent compared to other continuous heating systems but for an intermittent need, nothing will beat the comfort or cost of other radiant systems.
I have a just barely insulated drafty 70x35 metal shop building with a propane forced air heater suspended on chains. I don't have a tankconnected to it and have never fired it up. I have a few propane powered radiant heaters that "shine" a beam of IR to where I am working. I also have one of those big mushroom shaped propane heaters like you see on resturant patios. I tilt it 20-30 degrees to put its radiant "footprint" more to one side. All these heaters are portable and can be moved around as needed. I like the radiant heat because I can work without gloves in air that is so cold I can see my breath.
I am not familiar with brand names but a propane (or natural gas) fired heater with a blower to put air in with the injected gas where all combustion is inside an extended pipe that eventually transitions to an exhaust flue and provides almost pure radiant heat, these I have seen in operation and if I needed a true "on demand" shirt sleeve environment this is what I would want. The ones I have seen have an extended "linear" parabolic cross section reflector to direct the IR. Since they use a blower to provide combustion air it should be real easy to segregate the combustion air from the heated room's air, i.e. all combustion air would come from outside the building and all exhaust would go outside. No fumes, no draft/negative pressure problems and ZERO explosive hazard in a dusty environment.
This system doesn't have to be super efficient to be cost effective because it doens't heat the space ALL the time. Lots of folks like the vent free heaters but it creates a sutuation where YOU are living in the chimney. Some folks have bad reactions to the emissions. Im my opinion those devices are for intermitent use with a window open since they compete with you for oxygen and fill the room with lots of humidity as someone previously stated.
Patrick