The 22-250 case is too long to work in the AR-15 platform. You could build one on an AR-10 receiver, I believe.
Check out AR15.com, they have excellent tutorials w/ good pics on building AR's. As previously stated, you probably aren't going to save much $, but you will be able to build exactly what YOU want and with quality parts. Just remember, there's only two things on an AR that attribute to it's accuracy, the barrel and the trigger. All the rest is "lipstick and rouge".