Someone I work with changed out the upper on his Armalite, to a flat top, and said he'd sell me the used upper for $100, so I figured it would be a good reason to have a different gun. I don't need to shot a flea at 1000 yards, just want something else to play with. I found Armalite complete lowers in the $350-450 range, but not sure exactly what else I'll need to put one together. I understand I'll have to have an armalite lower, what about barrel? Anything else not brand compatible? Thanks