Armalite quality is A+++++++++. Customer service is reported as very good, although I've had no problems with my M15 A2. Thermold mags seem to be the favorite, I personally like the mil spec and colt 20 rounder. Hope this helps.
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Dont be surprised if thermolds wont hold your bolt open on the empty mag. Its not the mag, thermolds are excellent. Armalites are touchy about mags. Check the Armalite board out over at Industry forums. My M15A2 only likes GI or Colt mags.

Nice choice for a rifle... the M15A4 SPR was my first [^].

For a tactical sling I highly recommend the Giles sold by The Wilderness. They start out stiff but after a bit wear in. It’s very comfortable and very fast. They can be adjusted to carry hands free in your center of mass or like a traditional American or African carry.

How about getting a book? I'll recommend Jim Crews "Some of the Answer, Urban Carbine" a technique and tactics manual for the Colt (or Armalite) carbine.

For ammo find yourself some good Winchester and try out a couple hundred rounds for function. I wouldn't worry if you get failures during the rifle's first 50-100 rounds as things wear in. Then try some cheaper plinking ammo like S&B, Wolf or Baurnal.

Stick with USGI magazines. You'll need 20 rounders for shooting prone or off the bench and of course 30's for off hand shooting.

I don't know if the tactical carry handle requires you to change the hand guards or not. Jim Crews book will give you the reasons not to hang your support hand off of the front of the rifle, the Army will too. Your support hand is supposed to merely cup under the hand guards. Otherwise you'll tend to transmit flinch or recovery motions into the rifle stringing your shots vertically.

Get yourself a nice soft carry case to protect the rifle in transit and a gun sock for storage protection.

The rifle is a fine one.

Welcome Hans. Great weapon choice, it will serve you well.