As locogringo says, depending on your club you may not need an AR mag pouch. I have one but have never used it. You can generally shoot a stage with just the mag in the gun, and if not, a back pocket works about as well as an AR mag pouch for a rare reload on long stages, IMO. A maglink will work great if that's your bag, but it does make the gun quite a bit heavier.
Shotgun you may not actually need carriers or "twins/quads" rigs unless your club's shotgun stages are long. I use weak-hand load and use shell caddies but 98% of the time I don't need to use them, because my 590 holds 9 in the gun and 12 on the gun (butt cuff shell holder and 6-round side saddle) and about 75% of the time I don't have to load any more than what's in the gun. IMO it's so slow to load the shotgun (for me) that it's almost always better to shoot any available targets with "the other gun" (pistol/rifle/PCC depending on stage design and description) so I mostly just shoot my shotgun dry and transition. YMMV, but ask people in your local club and see what they say.
Definitely want good retention/tension on your pistol holster. IMO the Safariland ALS/SLS is a great system for cheap, but a good holster with tensioning buttons/screws will probably work. Running with a full high-cap pistol in a holster is a tall order without mechanical retention. You definitely need good pistol mag pouches if you haven't added those to your list.
When you're starting out, worry more about shooting safely and having fun than outright speed, and basically the only gear you really will probably need is a good belt, pistol holster, and pistol mag pouches.