Sounds like you need a turret adjustable scope not a red dot.
In order of my priority:
1. Basic cleaning kit. Something to either pull or push patches, brushes through the bore. An AR-chamber brush. Solvent & lube. I also highly recommend the CAT-M4 bolt & carrier cleaning tool. See the cleaning tool sticky thread.
2. A case. You'll need to transport your rifle.
3. Ammunition. Buy a case of Wolf Gold for plinking and practice.
4. Extra magazines. I've not known a Sport to be magazine picky. I own Magpul mags and D&H aluminum mags w/magpul followers.
5. As I mentioned before, for your purpose of use you'll want to mount a turret adjustable scope & rings. To answer the next question that is always asked, do not worry about the front sight post. It blurs out under magnification. You won't notice it.
6. Trigger upgrade. There is nothing wrong with the stock combat trigger, but a decent two stage trigger makes a world of difference. You can go all-in with a Geissele trigger or you can go to relatively affordable such as a Rock River Arms 2-stage varmint trigger.
7. After shooting with 5 & 6 and you've confirmed that you're shooting to the best of your ability and you want to try to eke out just a little more precision and accuracy potential, free float the handguard. There are drop in setups that are compatible with a front sight post. There are some that require more work.
Put some thought into points #6 and #7. If you even think you might want to do this, then reconsider the M&P 15-Sport II. You will most likely save $$$, time, and headaches in the long run if you purchase a rifle equipped from the factory with #6 & #7.
Be realistic about your intended purpose of use in relation to your marksmanship skill level, envision the AR-15 that matches your requirements. Go and buy a factory complete rifle equipped as many of the features you require that fits within your budget.
Take it from me, the slow AR-15 upgrade process is akin to a car salesman tricking a buyer into thinking only the monthly car payment and not the total cost of the car.
Do you like to play chess?
It's an election year, with Hillary as the Democratic candidate. The Supreme Court lost Justice Scalia. The next President will get to nominate a replacement for an evenly split pro/anti 2nd Amendment Supreme Court makeup. The recent
terrorist attack in FL has stirred up gun control and anti 2nd amendment rhetoric in the main stream media and in Politics. I am of the opinion that a panic run on all firearms and ammunition is a certainty. More citizens whom are not anti-firearms but never had a reason to own one will most likely go out to buy a semi auto pistol and rifle solely because the high probability that more of our rights to bear arms will be infringed upon. They'll want to buy the modern sporting rifle that is demonized in the media: an AR-15.
I've seen cyclical panic runs during the BHO's 1st and 2nd election campaign seasons. I've ridden out the post Newtown panic run on all ammunition. Panicked individuals temporarily disrupt the balance of supply and demand. Demand is temporarily inflated, supply temporarily can not meet demand, prices rise to whatever the market will bear. Panicked people pay anything. Fear feeds the panic cycle, and more people get drawn into it. An AR-15 that once retailed $500 will sell for $900+ during a panic. A 30 round no-name aluminum magazine once retailed for $7 becomes a $100 magazine. Ammunition runs start with a depletion of the common calibers first. I saw .22lr, 9mm, .223/5.56, & .308 Win go first. Then all of a sudden 22-250 and every other caliber is gone.
I'll see whining cries of "gouging", "unfair", and calls for borderline socialist / communist redistribution of ammunition, magazines, and firearms in the name of "firearms brotherood" from the unprepared as they pay the panic induced inflated market price for firearms and ammunition because they are late to the game. The panic cycle also draws in speculators. All of a sudden the average Joe Schmo thinks of firearms as a short term investment to flip. Speculative buying on credit. Some don't sell while the panic is full swing because of greed: they think if they hold out their profit margin will exponentially increase.
This is why I'm asking if you like to play chess (and like to gamble a little bit). If you're willing to take a chance on riding out the panic and take a chance on the anti 2nd Amendment election year rhetoric not materializing into reality, you can find great deals in the 2nd quarter of 2017. Typically, the best time to buy a firearm is about 4 to 6 months after a panic cycle when supply has stabilized. Credit payments are either due or past due. There usually are a bunch of new in box "used" firearms on the racks to choose from at low prices.
I've wrapped up my firearms and ammunition purchases for 2016 and am prepared to ride out the remainder of the election year. I'm now accumulating my firearms budget up to the 2nd quarter of 2017 so that I have the option to pounce on any outstanding post-panic deal, gem, bucket list gun that may present itself.