For an all-around gun-

Cons

-18in Barrel is to long and unnecessary
-6.8 is expensive and unnecessary
-Vltor Upper while being high quality, is heavy and unnecessary



What you described is really a specialized gun.



You mentioned using it for SD/HD. If you hadn't then it's just whatever you want. If you think you may have it for serious use then there are certain things you will want.

Stay with tier on manufactures-Colt, LMT, Noveske, etc., or a really good "smith" that uses tier one parts. S&W also makes a pretty good gun. Forget Bushmaster, DPMS, Rock River, Olympic, etc. for serious use. They have to many problems.

Make sure you get a barrel that is chambered in 5.56 with a 1-7 or 1-8 twist, do not get one with a 223 chamber. 1 in 7 or 1 in 8 twist will shoot light bullets just fine, and will stabilize 77gr OTM's which do make a difference at range. Make sure it has a chrome lined chamber and bore. You're not using it for High Power matches- a 5.56 chamber and fully chrome lined barrel will help make sure it works, and as long as you get a good quality barrel you will not have a problem averaging 1MOA with match ammo.
You can also use a good stainless barrel with a Wylde or MK262 match chamber, but you do give up some longevity and durability, and reliabilty with certain types of ammo with them.

An 18-20in barrel is unnecessary. It decreases maneuverability, adds weight and aids very little in mid range shooting. A 16in barreled gun is fully capable of shooting and hitting at mid range (5-600yds), and is much better for running and gunning.

Use a free-float handguard.

In general stay away from "match" triggers. If you shoot a lot and/or attend a training class, and you should, they will fail. The exceptions to this are the Gesielle and the AR Gold, though the utility of a 2lb trigger in self protection is debatable.

As to optics- they can be whatever you prefer. All-around means from contact distance to mid range (0-500yds or so) however in real life the mass majority of use is probably going to be under 200yds. So a suitable red-dot (Aimpoint) especially with a magnifier or a low power variable (1-4x, 1.5-5, etc.) is going to be the most useful.



There is actually a name for this type of rifle and it was started by military SOF. It's called a Recce, and they work very well at a whole bunch of tasks.