What would be an ideal heating situation for a small cabin [View all]

I built a small office/studio in the backyard of my house a few years ago. I've cooled it successfully with an AC window unit, but heating is another issue.

I have been using a radiator space heater which heats it fine, but when I lock up at night I have to leave it on unsupervised. That freaks me out quite a bit! There has to be constant heat since the PC, Tv's and guitar equipment stay there.

The size is about 20X20 (400 sq. feet) and fully insulated. If worse comes to worse a small heating unit with ducts could be installed. Any other ideas?

PS: I've seen window units that heat as well as cool, but the BTU ratings on them for heat is around 3800. Doubtful that can cover the colder nights, which can dip in the mid 20's Fahrenheit at the worst.

Thanks for any ideas