You'll be doing some semi-serious gunsmithing, so you should invest in a few tools. Most important is a vise block that lets you clamp your lower rightside up or upside down. You need to free up as many hands as possible. It makes all the other jobs easier.
Next, a combo wrench for working on the stock, and possibly the barrel extension. With either an A2 fixed stock, or A3 sliding stock, you'll need it. Also, many other weapons that use M16 magazines, like Sig, H&K, DPMS ACR, and a lot of .22 clones will benefit from having these tools around.
Third, get a hammer block to protect the lower whenever you do trigger work. You don't want that hammer beating up the thin web just behind the magazine well when the upper receiver is removed. There are lots of specialized tools that are worthwhile of the job is critical, or you just do a lot of the stuff. I have four AR15 platforms (five, counting the .308), and have done major mods to two more for friends. For me, the tools are an investment in ease of use, and getting it done right the first time.
As far as accuracy goes, two things, both in the lower receiver, are critical. One is the trigger. The other is some way to consistently lock the upper and lower together without either slop or excessive force needed to do the job.
Here's a shot of part of the lower receiver around the pistol grip. There are five different aftermarket mods shown.
The Demon Tactical rear takedown pin will benefit any AR15-type, regardless of other accuracy mods. It replaces the rear takedown pin, and allows you to snug up the final fit to whatever type of consistent pressure you prefer. Brand X uppers and Brand Y lowers may not snug up to your satisfaction, but this will let you "tune" it to anything you feel comfortable with.
Every time you break down the weapon, you may have to spend a few minutes with a small screwdriver (or even your thumbnail) on the left side of the pin to expand the bronze sleeve to your satisfaction. But once it's down, the two assemblies are clamped together consistently on all three axes.
Also in the picture are the Magpul pistol grip (great ergonomics), and Magpul fixed extra-large trigger guard. Gene Stone designed a great weapon, but missed out on some of the finer points of ergonomics. So did Mikhail Kalashnikov. And if you're buying all your fire control parts, pick out an ambidextrous selector if you like the concept.
Finally, this picture is of my "suburban defense carbine", based on the AR57 upper receiver.
The magazines with the 29gr hollow points have low penetration, but excellent wounding power. The 40gr blue tips can easily take care of zombies to 100m, which is more than their AK47s can do. I think the front takedown pin and the non-functioning bolt hold-open are the only stock parts in the lower receiver, just because I couldn't see a need to replace them. I like this setup better than my origianl PS90, simply because so much of it uses my 40 years of M16 shooting and gunsmithing experience.
If you can only remember one thing, remember "Brownells".