Just shoot the gun for a while, before buying anything for it. Learn the gun, then dress it up, if you feel the need. I like red dot optics on my ARs; there a lot of choices, but I prefer Aimpoint or EOTech.
As far as ammo goes, any good quality 223 Remington or 5.56 NATO will work fine.
I wouldn't waste my time breaking in a barrel on a regular AR like it is some kind of super-accurate bench rest gun. That just sounds like a bunch of gun store B.S.
Here's an excellent thread about cleaning/lubing an AR:
http://www.m4carbine.net/showthread.php?t=35490
And here's two of my AR's, a Colt 6920 on the left (with an EOTech 512), and a Daniel Defense lower with BCM midlength upper on the right (with an Aimpoint H1).