Joey, you might get a hold of Taran Tactical and see what they suggest for the M&P 15. Me personally, I would probably leave it alone and upgrade a little better rifle. Sorry, had to say that. My favorite is the SIG MCX.
There were a couple of versions of rifling on the M&P, 1 in 8 and 1 in 9. The 1 in 8 is a bit better, but for tactical playing, probably not much difference. Most of the Sports have 1 in 9. Newer ones have 1 in 7 twist.
Also, there were California Compliant and non-compliant versions if I recall correctly. The California one has the infamous "Bullet Button". If it's an older unit, it may not have that.
As for tactical mods, think of things that will make it easier to get on target and feel more like a natural extension of your body. Forward grips are nice, but now illegal in California. Retractable butt-stock is a good way to tailor the stock length better to suit you (also illegal in California now).
The purpose of rails, whether they be single, triple or quad, is so you can attach enough goodies to the rifle (so you look really impressive to the other shooters). Foreward grip is one main purpose for the rail. The only other thing I would mount on the rail, is a flashlight, mainly to free up your hands (to stay in ready position on the rifle) when you need a flashlight.
Myself, I prefer to have EOTech sights, with backup angle/flip-up iron sights. The EOTech Hybrid Holographic are tops, but very pricey. Go to a local shop and try out a few red-dot style sights, and see what works with your visual sighting, in your price range. The cheaper sights, are usually just that, cheaper in all ways. The Aimpoint Micro T-1 is another favorite, and a hell of a lot cheaper than the EOTech Hybrid. Angle or flip-up iron sights are not really a necessity for modern optics (most modern optics are reliable as ever), but might be a consideration for your own desire. I prefer to have them. I run flip-up iron sights on all my rifles. Just in case that electronic optical decides to shut off for some known reason (Murphy's Law is alive and in full force in most tactical situations).
For slings, I prefer just a single line safety cord. Besides carrying the rifle all day in a ready carry, a sling has little purpose (although some shooter training likes to use the sling in a brace position, I would prefer to not have to rely on a sling for proper shooting position). The main purpose of a single line cord, is to keep the rifle close to you should you fall or for some reason, lose grip of the rifle, with the cord, you can retrieve it faster.
Also, check out training from Taran Butler (Taran Tactical). It's pretty awesome, and teaches more than just shooting a rifle.
I hope all is good with you and family, hope I said something that helps with your rifle.