I looked into having a central unit put out in my garage. It was going to be very spendy. My garage is a 3 car garage and it is oversize with a small workshop area on one end. I only really care about heating or cooling one bay and the workshop area. I did not want an open flame propane or something I had to vent so I went electric.
It is 220 and will heat up the area in 10 minutes to be totally comfortable to work while wearing a t-shirt and jeans. It just cycles on or of as needed. Plug and play installation.
For A/C I put in a window unit in a frame I built for the human size door that goes outside.
I can put the frame in the doorway and slide the AC unit in place and still close the steel door when the AC is not in use for full security. I put it up in the spring and take it down in the fall. It will keep me comfortable on days that are 100 degrees. Many times I have gone into the house and it noticed the garage was cooler than the house on a very hot day. I have calculated that a full day of AC cost me about 8 bucks. To be able to work all day and not be covered in sweat and exhausted from heat is worth every penny.
This is my AC project.
http://forums.pelicanparts.com/porsche-911-technical-forum/422393-my-garage-upgrade-project.html