While it would be great to have insulated walls, they are a smaller contributor to the heat gain in your garage. If the door and ceiling are insulated, all you need is a 110v window A/C unit. You'll need to cut a hole in the outside wall and add a dedicated 15- or 20-amp breaker to power it. If you're cheap and lazy like me, add a switch next to the door leading into the house to be able to turn it on and off without reaching the control panel (A/C units work best mounted high on the wall).
I cool my 720 sf garage with a 12,000 btu unit that runs on 110v. According to the experts I should have a bigger unit, around 18,000 btu but that would mean 220v. In addition, running too big a unit chills the air without dropping the humidity. I only run the A/C when I'm working in the garage and I've had bare steel exposed on the Corvette frame for almost 4 years and it doesn't even have surface rust.
A 12,000 btu 110v LG unit is around $350 and comes with a remote so you wouldn't need the switch by the door. I'm sure there are lower prices out there but you aren't looking at thousands of dollars.
http://www.livingdirect.com/lg-1200...onditioner-550-sq-ft/LW1215ER,default,pd.html