Wow, it's been a looong time since posted in these forums, and then I'm usually on the gardening ones LOL. I searched the forums but didn't come up with anything that answered my questions.

I'm planning on getting a free-standing 12x24 shed to use as my studio/workshop space and storage. I do a lot of furniture re-purposing and repair/refinishing, crafty type projects, and artsy stuff. I'm planning on insulating the walls, floor and ceiling as much as possible and caulking/sealing gaps to make it as efficient as I can to heat/cool. I won't need it to be 72 degrees year round, but I will need it to stay around 50 in the winter (paints and solvents will be stored there), with the ability to boost the heat up quickly when I'm actually out there working. In the summer, I'd like the ability to cool it down and take the humidity out just during really hot/humid spells, or when I need something I just painted to dry in a reasonable amount of time. :) I'm considering options for heating/cooling systems, with the most important factor being the operating costs...Hubs is paying for the shed, but I'm paying for the electric/gas bill! Here are the options I've come up with so far:
1) window AC unit + electric space heater (I don't like the idea of running a space heater when I'm not there, but it would be necessary to keep stuff from freezing)
2) window AC unit + upgraded (i.e. more permanent) electric or gas heater. I'm thinking a Blue Flame heater or a wall mounted electric heater that could safely run while no one's there.
3) one of those ductless wall mount heating & cooling units with the condenser (like they have in hotel rooms). I know the up-front cost would be more, but it would be worth it if it's more consistent, and more efficient.

I should note that I don't want or have room for a wood stove, or one of those garage heaters that hangs from the ceiling, or anything really smelly or noisy like a torpedo. Those are fine for hubby's garage, but won't work in this situation! Also, I would rather NOT run a gas line to the shed, but I would if it was going to give me huge savings in operating costs.

I would like to hear others thoughts/opinions/experiences with these or other heating/cooling set-ups, and I'd really like to know how to figure out how much (roughly) it would cost me a month to run one of those ductless units (couldn't find anything online).

Thanks ahead of time for your help!
Jill