Nice insulated garage! I'm jealous. I have a metal skin shop (Quonset hut style). It gets the job done, but is cold in the winter and hot in the summer.
Its important to note that you have no ventilation air into the space. Spray foam insulation is excellent at sealing every little crevice. When using this type of insulation, its a good idea to have a fresh air ventilator (with heat recovery) to help maintain good air quality. This will be especially true when doing things such as welding, brazing, torch cutting and so on. With the doors closed and well sealed, it is very possible to get into a toxic environment when working in the space. If you ever plan on operating the vehicle in the garage with the door closed, you should invest in the exhaust transfer ducting to slide onto your exhaust. They often have a booster fan inline.
something like this will get the job done:
ERV This size is likely overkill for your application, but would ensure good air quality. It would take about 60 hours or so to turn over the air in the space (assuming it's 20X20).
If you are worried about operating it continuously, you could get fancier and put it on a CO2 and/or CO sensor. For CO2, I would cycle it on at ~700 - 1000 ppm. For CO, I would cycle it on at about 10 PPM.
If you get a 'total' energy recovery, you don't have to worry about humid air as most of the ERV units are better than 90% efficient. If you buy the sensible only style, you will end up pumping a bunch of moisture in there when it is humid.
