Excellent choice on the M&P15. Not the best rifle, but definitely a good value. It'll serve you well.
I think you may find the ACOG to not be the best for your needs. They have integrated magnification, which is excellent in a primarily counter-sniper type of role. However, it may not be exactly what you need.
Considering you're looking at CQB, out to about 300meters, an ACOG is really over-kill. For CQB, it is easy to get tunnel vision on that magnified ACOG. You're also paying more than you really need to.
I would recommend either an Eotech or an AimPoint red dot/reflex/holographic sight. They don't have magnification. Personally, I prefer Eotech, but a lot of people like AimPoint.
Although the basic optic is not magnified, to get that 300meter range you can add a 3-4x magnifier behind it. Eotech and Aimpoint both offer magnifiers. Aimpoint has a twist-off mount, or a flip-aside mount. Eotech offers a flip-aside. The magnifiers are interchangeable, it doesn't matter who's you buy. I like the flip-to-side mount for the magnifier.
This allows you non-magnified CQB, but by just flipping up the magnifier you instantly get the counter-sniper set up for longer range. This set up will cost you around $800-$1000, just a little less than an ACOG, but with a lot more versatility.
Try to find a shop that has all of these, or try a gun show, see what you personally prefer. I like EoTech because the AimPoint reticles never look right to me. A whole lot of people prefer AimPoint, though. Both are great, as is the Trijicon ACOG, it is mostly preference.