re: Is it spring yet???

I use one of the oil filled radiators in my green house.  It isn't insulated (just thin poly-carbonate panels) and it will keep the green house in my set range between 52-57 even when the temperature gets down to the low 20's like it did this week.  My unit is uses about 1500 watts on high.   For a larger shop, you might need two or even 3 of them but they would probably need to be on separate circuits to keep from tripping the breakers.  A fan on low blowing on the vanes will improve heat movement.  

Before I added a minisplit, I used to have a portable AC with heat pump function in my shop that would keep the shop temperature in a workable range on all but the coldest days.   For a portable AC/Heater, it needs to be a double hosed one so that it doesn't suck in cold air to replace the exhaust air.  If you have a window, a window unit will be more efficient.   I also had a radiant heater positioned over my work bench that would  really help because the radiant nature means that you instantly feel the heat if you are in front of it.  I only ran that when I was out there but coupled with the portable HVAC unit, I could pretty much use my shop year round.  When the portable unit broke (I got it used and it lasted over 10 years), I replaced it with a mini-split which is a game changer.  The minisplit is the most important power tool in my shop.  I can use my shop year round even when the outside temperature is 110 or 15 degrees F and I have no  insulation on half of the walls, except for sheet rock, and the metal garage door just has 2" styrofoam insulation.

BTW, if you have any wood glue or water based finishes out in your shop, you might want to bring them inside.  Some of them can be ruined if they freeze.