I found this - PSA AR-15/M16/M4 Ultimate Lower Parts Kit w/ Magpul, Mil-Spec. - kit for a lower receiver for a M4. I was wondering what you guys thought of this kit. What would be a good Upper Receiver to match it, I wanna go with 5.56mm. And Also, how hard is it to actually build a AR-15 from a kit. I've never built a gun before, so this would be my first, so I was hoping for some hints and tips on getting started, if I do decide to do this. And if anyone who's done it, good give me some details on their experience building an AR-15 from a kit.