Old but good string. I had an evaporative cooler for some time in my 12x24 shop/shed. Later swapped it for a 6K AC window unit that goes through the outside wall. But I also have a lot of insulation in place in the ceiling and walls. What has really helped a lot is laying in a layer of radiant barrier (aluminized bubble wrap), then the insulaiton. When we insulated the garage, we measured the heat difference between the uninsulated roof and the spot next to it with the radiant barrier. The covered roof was 35 degrees lower. Good stuff, have it in the garage door too.
With all the insulation, about R20 in the walls, a ceramic heater holds the temp nicely, but it takes two heaters to bring the temp up.
If I had it to do over again in my 2x4 walls, I'd have added 2x2s on the studs so I could have put R30+ in the walls. In other words, I come down on the insulate like mad and use a decent sized AC. You could probably find a used AC unit if you searched a bit.
BTW, I live in the high, dry desert, and didn't have much rust problem, but then I also use boeshield rust inhibitor on all my cast iron. With the AC, I don't have to concern myself with condensation.
The more I do, the less I accomplish.