Grizzly,
The Rifle will be a Primary defensive arm and used for training to improve on that. I am not LEO and there is little to no odds on being called into having to use it in anger...barring a Canadian invasion. Why Duty grade, because if you will have one rifle why not have it be the best it can be right?
I've done several short stints alongside LEOs and .mil guys locally but nothing in a formal class yet. In those I was using a Kalashnikov so I'll be having to retrain to operate the AR. Ammowise is it worth the extra savings to use brass case vs steel? I want to say yes.
The lower needs some more rounds. I have thought about using the DPMS lower as a secondary build and pinning on a SOLGW full lower. If reading the thread on the Facebook group this would then make a full Duty Grade?
I'm Set on a Aimpoint. Ambi safety on that who is a good option on that?
Rail. I currently have on the left side a QD mount with plans to place the Flashlight mount on the far front right. You have me on the X rail vs SL.
Sights- I would not mind a finer sight picture but then these are BUS and Aimpoint hardly will go down. I suspect that metal would be more about aesthetics rather then function in that manner.
First off I'm not an expert compared to many of the very smart people around here. But my thoughts on what you are looking for is this.
First, set up a basic rifle. From all I've heard SOLGW is good to go on quality but double check the upper and BCG. there are numerous threads about checking for the big things (feed ramps, staked gas key, extractor springs etc etc). Then double check the lower to the same standards (buffer, spring, fitment etc). I don't know much about ambi safeties so I'll skip that question.
Once everything looks good on the inspection add what you need to setup a defensive carbine. To me that would be:
A good light with at least 500 lumen in a mount that keeps it close to the rifle. If the light is much under $100 it probably isn't that great of quality. I'd say streamlight or surefire here.
Any dot in the aimpoint family.
A two point sling.
10 good GI steel mags or pmags (let the religious debate begin).
Magpul MBUS or MBUS pro.
Now take that simple rifle to a class that is from a vetted instructor. The P&S network has great info on this. Don't go to the gun show and sign up for a class from a "navy seal". Get a vetted and reputable instructor. Your local guys might be great. Pat Mac, Fisher, DARC etc ARE great. Don't skimp on your class anymore than you'd go with a sightmark red dot instead of an aimpoint to bet your life on. Bring a notebook.
Now after the class you will have a very good idea of what changes you want on the rifle. Chances are they won't be much. Depending on your arm length getting a rail that goes farther down the rifle might be worth the money to you and maybe it won't. But you'll know AFTER the class what works for you and what doesn't.
IF (IF IF IF) you have the money to do a class (ammo included) AND buy another lower you might be well served to pick up a BCM, SOLGW, Scionics *SP* or other quality lower to have as a backup for class in case your existing one goes tits up.
As for ammo I've got thousands of rounds of steel through mine and its been fine. Odds are good it will hurt you if the class you go to stretches out past 200 yards. Wolf (even gold) simply isn't made to hold the same accuracy standards as match grade ammo. Zero your rifle with your self defense ammo. Be sure the rifle feeds well (100+ rounds ) of your self defense ammo. Then if you do the class with mostly steel you might want to confirm zero at distance with the steel ammo and rezero the rifle with steel for the class if needed. Then be sure and rezero with the good stuff.
