Basically, a frost guard should be enough. I also have one running in my garden shed because there is already the kitchen and plenty of moving boxes inside. That was enough even during the very cold period to keep the garden shed above freezing, although the frost guard is actually undersized. 500W frost guard, house is 3.5x4m with a gable roof made of 45 mm log boards.
However, I find it a bit off to build an efficient house and then permanently heat the shed with electricity. For paint, etc., I would therefore use an insulated cabinet with a frost guard. Tools without batteries usually don’t suffer from the cold, batteries in the insulated cabinet. Beer can handle the usual 3-5 degrees below zero; during severe cold waves, I would just bring it to safety in time ;)
