My gun would be mostly used for punching paper so I'm interested in accuracy.
The barrel is the heart of your accuracy potential in an AR platform. Look for one made of stainless if possible. The Chrome lined barrels are great for "fighting" guns and some are accurate but you stand a better chance of getting a more accurate rifle if you bypass the chrome.
As others have mentioned a good trigger will go a long way in helping with accuracy. The Timney is a good choice. The best I have ever used is the Geissele Hi-Speed models.
A model with a free float tube or rail system will be very helpful in getting every last bit of accuracy from your rifle.
I prefer a minimum barrel length of 18" for an AR that I might ever decide to hunt with. The 14.5" and 16" barrels are nice but I find them too loud to shoot without hearing protection and I never wear that when I hunt. For a fighting rifle the 14.5" (with a pinned F/S)is an excellent choice.
I'm sure you've already figured this out but get a flat top upper.
If you're not building your own rifle you don't need any special tools. They clean up just like any other rifle. Just keep the bolt carrier group oiled and you'll be good to go.
