Re: optics for ar build
Leupold mk4 1.5-5x MR/T w/illum SPR reticle is a good choice for your goals. Compact and relatively lightweight, the scope will also enable Bindon Aiming Concept techniques with reticle on and flip-up covering objective lens.
For sniper/spotter events though, why go optic at all? An AR in A2 Service Rifle configuration would be lighter and faster to deploy. A Leupold AR series 1-4 will mount very low on an A2 carryhandle if you want, find you need a scope.
You want the 20" barrel with 1:7 or 1:8 twist and are planning a freefloat handguard, right? 69 or 77gr match ammunition will easily fulfill your goals to 400yds or way longer. Buy an A4 SR upper and have the best of both worlds, since the A4 SR upper will come with detachable carryhandle rear and front NM sights you can remove when you want to go flat top.
Putting a large and heavy scopesight on an AR-15 is an encumbrance. On an AR-10, maybe. If you are backing up a sniper with this rifle you want fast reaction, precision sights and some balance and weight to enable fast shot strings; don't you? Put the extra weight into heavy barrel w/free-float tube, learn to use NM iron sights and don't look back. Not much more you will need.