I have a line on a pre-ban Colt AR NIB through my father-in-law. What sort of concerns should I have about buying this? I really like the 6920 (it being a blank slate, well made, and I can put all of the add on's I like on it). Will a model like this give me the flexibility to add sights, etc? Or will I need to have modifications done? Also, how much should something like this cost?