Well body shops seem to be hit or miss. They either **** ity up completely or you couldnt be happier.
So on that note I will just look around your area for a few local shops with a good reputation. Ask people at car shows where they had theirs done. When you do find a place have a look at their previous works. See how many mustangs they have done.
When they are doing the work, as annoying as it sounds, constantly be stopping in to see how its doing. Some of these local shops around here have a rep of overcharging labor. This also kind of keeps them motivated to do a fine job because you obviously care how things are going and if your not happy your going to nitpick and such. They just want their money.
Price is all gonna depend on how much body work the car needs, the type of paint used, and how much clear coat they apply. Just figure out what you want and talk to people.
One word of advice..
In paint, you get what you pay for. Don't skimp on quality cause the price is right. You'll regret in when you can see scratches you can never get out because they are under the paint in the primer. Ala lazy painter that didn't sand fine enough. Or the good old fashioned overspray on the cowl/tires/headlights. PAY FOR QUALITY!