For the basic platform, Colt's Manufacturing Company, Inc. is your best buy. Unless you plan to use this rifle in sanctioned competition, you haven't any need for the many expensive modifications available from a laundry list of suppliers and gunsmiths. Having toted Black Magic for three years as an infantryman in the US Army, I can bear witness that this rifle can take the most egregious abuse with a smile and keep on operating. Don't allow the barrel to heat up, keep lubricant where it belongs, don't use the alloy receiver to hammer tent stakes, and strip the bolt down with each cleaning. Hunting for lost pins and springs is no fun.
Chrome lining the bore is a waste of money.
Chrome lined bores are found in SKS rifles and some other Russian, European and Chinese military weapons. The purpose is to keep the bore from rusting and pitting from the use of corrosive primed ammo. I haven't paid much attention to 'hot deals' on imported 5.56 NATO ammo loaded with corrosive primers, so I can't say whether this is something to be vigilant over. Make sure, to be on the safe side, that any ammo you buy is marked 'Non Corrosive'. The AR15 isn't much bothered by punishment, but that doesn't mean you should neglect it.