I'm "building" one right now. I picked up a RRA stripped lower, and am adding parts to it. I plan on buying the RRA Predator Pursuit upper, assuming I can actually find one.
As for magazines, the Magpul PMAG
seems the way to go. They are virtually indestructible and cost around $14-16 a piece for the 30 round variety. Right now, you can find the 30 round mags for sale at various places. Go to ar15.com, and under the ar-15/fourum, there are threads on magazines and where they have stock. I have heard that the 20 round magazines will be back in stock shortly. Here is a link via ar-15 (AR 15/M16 magazine PMAG 30 Round
Let us know how your rifle shoots!