That is affordable target ammo. One thing though the 556 version of this ammo shoots differently than the 223 Bronze. My VSSF II 26" shoots the 223 stuff one small hole at 100 metres. I was very surprised. The 556 version of the PMC is the same ammo but in 556 and shoots about 1 moa. This rifle is 223 chambered though. Yours is the accurized 5.56. Also the twist rate on that Remington was 1:12 so you may need to go 62 grain or higher for best performance since yours is 1:8. Use the 55 PMC to get on paper and get used to shooting the rifle. Then look for the 62 grain PMC along with Federal gold 69SMK. Reloading after that. I know some have luck with Hornady ammo. Personally none of my firearms shoot Hornady ammo. So I avoid it like the plague. I suspect if your rifle can stabilize it that the 70+ grain stuff might be decent. If you have the right twist rate and can find that ammo.
Armalite makes a decent product. I personally think they are very under rated for what you get. The upper and lowers of the new ones are the same as the Aero precision forgings which many rave about on here. The older ones didn't have the same fit/finish.
The most direct competition for the rifle you bought would be the Stag 6. It's a tough call which one to get. The main difference would be barrel length. I would highly recommend either rifle.
Do yourself a favour though and order the Magpul MIAD for that rifle to replace the pistol grip. It's the cheapest and best upgrade you can make. I like the ergo grips as well, but for a target rifle the bigger grip for the MIAD makes bringing the trigger straight back a bit easier.
The A2 stock will work fine. If going to upgrade then look for a Magpul PRS. But in reality the A2 works very well. I haven't bothered changing the ones on my AR10 rifles since they do the job nicely.
The 3-9x will work for 100 yards. You can do ok at 200 yards. After that you'll be wishing you had more magnification if target shooting. That being said since you have a 20" barrel you can still use it off hand and for general S&Gs. For a dedicated target rifle I prefer 20x or more.
Congratulatons. That's a nice rifle. Consider getting the matching one in 308. The AR10 takes it up yet another notch.