The first thing to do with any weapon is to break it down as much as the manual suggests and check it for bad parts / badly installed parts - etc.. You don't want a gun that has a part installed wrong that may blow up in your face. I check "every" firearm I buy before I shoot it.

There are better types of ammo for target shooting. Target shooters generally roll their own (re-load) but there are good brands off the shelf like Black Hills ammo. Every rifle is different. If you want maximum accuracy I suggest trying several different types and brands of ammo to see which shoots better in your particular rifle.

For the first AR I'd say buy one pre-built like the Sport II. The warranty alone makes it a good deal. And you won't get burned by a badly built rifle (which I have been although it was an AK and not an AR). My second AR was built by someone but by then I knew a lot more about what I was getting and what to expect.

I wouldn't worry about changing parts until you shoot a lot. You may find that you don't need to change anything. The only thing I changed on my Sport was the handguard. I wanted one with an actual heat shield. That's probably the weakest part of a Sport. Some say to change out to a free floated design. My Sport shoots really accurate with the stock setup. I can shoot about a 5" group at 135 yards shooting off hand with my Sport. It's going to be a challenge to get one to shoot better than that. You may make it worse instead of better. If the accuracy doesn't suit you as it comes then you might go with a free floated design. I have other rifles that are certainly more accurate because of being free floated but again my Sport shoots plenty good without that. I've seen reviews that say it's going to be hard to make a Sport shoot better just by changing to a free floated design. I'd have to see it to believe it to be honest.