I have worked in every type of garage known to man. I also have LOTS of experience insulting! (everything from closed cell foam, rigid boards, cellulose etc etc)
If you want to be comfortable at ALL, you NEED to insulate that thing first.....The MORE the better!
If you are on a budget, fiberglass bats(though rock wool is WAY better, rock wool R value does not decrease with temperature like fiberglass) in the walls, and cellulose blown in for the attic is the BEST bang for your buck. If you buy 10 bags, you get to use the machine free.
https://www.homedepot.com/p/GreenFi...w-in-Insulation-19-lbs-Bag-INS541LD/100318635
You can get away with one of these for heat to start, but because it is ventless you WILL get lots of moisture in the garage....not recommended to run these 24/7, but work GREAT while working!
https://www.homedepot.com/p/Dyna-Gl...gclsrc=aw.ds&dclid=CJvP1oCv-N0CFYFpwQodtE8NaA
I run one of these in my 30x12 single car, it heats it up no problem. 100 bucks a month to keep it at 65 degrees 24/7 all winter long in NH
https://www.northerntool.com/shop/tools/product_200578579_200578579
I'm currently drawing up an in floor hydronic system for my new garage.....lots of planning involved