Dude!! You need to decide what you want. You could just buy a Bushmaster upper from www.bushmaster.com or www.impact.com. or www.classicarms.com and look at Rockpoint Guns.
Also you have to decide what you want to use the weapon for; as in M-4 shorty adjustable stock for house entry and clearing. A target 16-20 inch barrel open combat or a bull barrel varmint style for mid range sniping. Once you know what you want it will be easier to build an upper.
Are you married? ;)