[Moneyguy1]

Running the cooler when you aren't in an area is no problem if you install three-way switches in two locations. An insulated door will help, but you will still need some ventilation. Your bedroom is right above. So, I assume you mention this because a ceiling exhaust is out of the question. What is wrong with a wall exhaust? Is the floor space between your bedroom floor and the ceiling of the garage insulated? Are the garage walls insulated?

You say 600 square feet and I assume 8 foot ceiling height. That is 4,800 cubic feet of volume. Armed with that number and info on insulation, any A/C dealer worth his salt will be able to size a wall unit for the space, given the amount you want to drop the temp. I would assume that on a 110 degree day, the mid eighties would be your goal.