If you're not going to have ventilation (which I'd reconsider .. stain and polyurethane are pretty stinky materials to use, a lil air might help you avoid a nasty headache and potential health issues from inadvertently huffing) an electric heater would be best. Kerosene heaters are going to produce additional fumes which is going to add to an already smelly situation. I'd go for an electric model and keep it as far away from the piece of furniture as you can. Here is a good page on electric heaters. I'd seriously consider having some sort of ventilation though just because of the odor and potential fumes if the stain and poly get warmer, they're likely to give off more fumes than if they're cold.
First off, as long as the temps stay in range, I don't think you'll have a problem with an 'air-tight' container. The only reason computer equipment needs airflow is because it's dumping heat into that air. As long as you have a little bit of air, and can keep it at an acceptable temp, low airflow won't be a problem. You can't go zero airflow because the equipment in question isn't built to heatsink to it's case.
Off had the first thing that occurs to me is a 'garage fridge'(something like this: http://www.amazon.com/dp/B001B2P2G8/?tag=googhydr-20&hvadid=3788940351&ref=pd_sl_6z7wf02xh3_b)
The thing includes a heater system to keep it's refrigerated section from freezing in cold temps.
That's really more than you need though, because it's going to try to keep stuff cold when you really only need it to keep stuff room temp. It's going for a much narrower temp range than you'll care about.
If I were going to try this, I'd get an old fridge (doesn't have to work), and build my own controls (some kind of micro-controller) - I'd cut some kind of vents with controllable louvers and fans into the top and side. And some kind of heating system (really small space header? dunno). I'd add in a temp sensor, and turn the heater on when things get too cold, and the fans on (with louvers open) when things get too hot. If the temp gets out of range, cut power to the device in question.
If you want it to deal with hot days as well, then get a fridge that WORKS, and put the chillers it into the mix as well - if things get hot enough that the fans aren't doing it, cut them, close the louvers and hit the chillers.
Best Answer
I would completely disagree with your builder. Since it is a new garage, I do see merit in the "build it in" approach. There are two important considerations:
how many CFM do you need and what kind of noise level is tolerable? Are you using anything flammable? Water based paints should not be a problem but solvent based paints require special attention. This will drive your fan selection. Of course you want to make sure that the way the fan is turned on is convenient to you. Would you want to have more than one switch? Perhaps a motion detector controller would be even better? Now is the time to choose.
you must have a supply of "make up air".
How will new fresh air come in to replace the air you vent? Venting will be most effective if you can arrange a cross flow through your work area instead of a short circuit or whatever might happen without a proper design.
It may be worth it to run a duct just to force the air to flow across. If you wish to get fancy, you could incorporate some degreee of filtering as well.
This is outside the scope of your question but I always like to make sure a work area is well lighted with plenty of lumens and as little glare as possible. Since you are painting, a good (high) CRI is probably important also.