I'm thinking of doing the same. I've been told in the long run purchasing the prebuilt ones will be cheaper but the problem is the tallest prebuilt one for a decent price has a roof that is 6'2" and I'm 6'7". Someone posted on a website to buy one of these kits for $50 and it will really save some effort. It seems like you don't have to worry about angle cuts so it would save some time and effort.

http://www.northerntool.com/shop/tools/product_572_572