There are a lot of factors to consider: 1) how much do you want to spend, 2) what level of comfort do you want/require, and 3) the hassle factor. The first is obvious - your call. The second is more subjective but most people can work comfortably at about 60 degrees. I prefer it a bit warmer. And the final one relates to the first and what's available in your area. For example, do you have access to gas? Are you willing to put in a propane tank?
Five years ago I too was in your boat. In my case, I don't have gas and didn't want to go the propane route even though I have a propane fireplace. It was easiest and cheapest for me to install at 75,000 watt King Eco 2S electric. The entire effort - the unit + paying an electrician - was less than $1,000. I am relatively comfortable in my 700 square foot garage with 11 ft ceiling. Because of the ceiling height, I have to run fans to keep the air moving. Otherwise, all that hot air is above my head and doing me no good.
On the other hand, if I had to do it over again, I'd likely splurge and go with a mini split. Remember, that's a heat pump and you're rewarded with summer time cooling.
