I converted a dirt-floor pole barn into a nice shop. It is 24 x 48 with an L wing added on to it that is 16 x 20. I know I'm from the South, but we get some reasonably cold weather down here as well. (Tonight, we're headed for about 23 with a high tomorrow of 32.) The building has metal siding with a shingle roof, the entire front side (48') has four garage doors on it, there is no insulation on the inside, and the ceilings are exposed all the way to the gables. I had a natural gas forced air heat system installed by a qualified HVAC guy. He installed vents in all the critical work stations, and it works great. I turn the system off at night and only use it as necessary. On the absolute coldest days, it can stay pretty 'cool', but I've never had to stop working because it was just too unbearably cold - and it's never necessary to wear a jacket while working. Usually, I'll need to manually turn off the heat while I'm still working because it gets too warm. In the farthest corner of the shop I had a vent fan wall unit installed to keep the air moving. (AC would be nice down here, but not very efficient in my type of building.) The real trick to making the system viable is that the heat exchanger is in the 'attic', that is, up in the gable. It takes up no floor space, and it has a filter element that is easily accessible. If your shop is like most, you'll need to keep an eye on that pretty regularly. Obviously, the system is not as optimal as a home unit with proper insulation/sheet rock/etc., but it's really nice for a shop. You need to be comfortable, but when you're working, it takes far less heat than when you're resting. I tried the space heater approach with very poor results (you're not always going to be that close to the centralized heat source). Bottom line: I've been very pleased with this setup. I live in a farming community, so I have several friends who, like me, have converted barns into shops. NONE has a shop as uniformly comfortable in cold weather as mine. If you can get a forced air system installed without using up floor space, give it some serious consideration. A good - and honest - HVAC guy can help you decide it it's right for your application.
Regards,
Terry