One thing to think about - I doubt anyone here would disagree...

Instead of trying to get everything you need/want on one rifle, realize this is a disease and you're likely to end up building another upper if not an entire rifle at some point. So for example, build up a good 16" carbine M4 setup the first time and shoot it up and enjoy it. Then figure you're going to want to put together a 20" govt profile setup at some point for "long range" or whatever. If you're unlucky, you'll get bit by the NFA bug and decide you can't live without a 10.5 or 12.5 short barreled upper too, with a noveske pig - or eventually a suppressor. Maybe a slide fire stock or a 22lr conversion kit (or a 22 upper). Plus now that you have 3-4 uppers, someone's going to point you to a sale on stripped lowers, so you might as well build up the lowers to match all those guns, too. And then you'll have several rifles, a box of leftover parts that you'll be trying to sell/trade on the boards just like the rest of us, and you'll be eating ramen because you can't afford steak anymore.

LOL - really though, my only point is not to stress over it too much. Most guys when they build their first rifle spend a ton of time stressing over the parts. I'm betting everyone has been in your shoes at some point. The good thing about the AR platform is you can always switch it out, and you're likely to keep messing with it as you go anyway. Pick a good set up, or copy someone's and get shooting. SAVE SOME MONEY FOR AMMO - it's still stupid expensive. Lots of good advice here and on other boards, but nothing will be as good as your own personal impressions and needs once you get to shooting your gun.