As someone who works out of an unfinished basement, I like this question. A few years ago I had someone install an electric heater that’s about 24″ x 16″ or so. It has a fan in it and is located about 12′ from my workbench. It’s not that powerful but it takes the chill out of the air and is usually adequate for winters in upstate NY. If it gets particularly cold, I’ll also use a small space heater. I can pretty reliably get my work area up to around 60F in the winter, but not much more than that unfortunately. Additionally, I’ll use a humidifier to bring up the ambient humidity from as low as 30% to something closer to 40%.
The extremes of my shop that I’ve measured are around 80F and 70% in the summer, and 40F and 30% in the winter. This is if I leave things unattended which I obviously don’t. Essentially, it’s a balance between getting a decent temperature and humidity range in the shop. I’ll often keep customers guitars upstairs and out of the shop if the conditions are more favorable. This time of year, I feel comfortable leaving guitars in the shop. I think what’s most important is to be mindful and do the best you can.
Edit: Just to answer a couple more questions, I rarely heat overnight unless it gets particularly cold out (single digits/below zero) and I’m worried about pipes freezing. When it gets below 60F is when I start to really feel it. I’m not a big guy and so I have to dress particularly warm. One thing that’s worth getting is fingerless gloves. Lastly, my honest feeling is that since the customers guitars are not in my possession for longer than a few weeks, I don’t think they’re likely to be ravaged by temperature and humidity, even if the conditions are less than ideal. I am mindful of the environment and do the best I can, but I also don’t feel that guitars are as delicate as one would be inclined to think.