So you have the carbine length gas system? What front sight do you have? What kind of rail do you want? FF or will you just get the handguard replacement style? Gotta start reading (stickys), and more importantly shooting, at least a few hundred rounds before upgrading parts.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek