You shouldn't need more than a quart to do the job but it really depends on the gun. I'm not a pro though so I'm not the most qualified person to answer this question.
In my opinion this is one of those times where it is best to go to your local paint guy and ask him. When compared to ebay or mail order, the extra amount of money that you'll end up paying him for supplies will be more than made up for in advice and guidence. Those guys know thier products, and once they know your project and experience level, thier recommendations are priceless and in most cases they won't BS you. If you find that you can do this project, you are more likely to do another one, and that means more sales for them.