For the kind of money you are willing to spend you can get a very decent 5.56 AR15 rifle there are many good choices among those already listed.
Research your options and plan out your purchase if you shop wisely you can have a well balanced and thought out practical package.
Options to consider are the flat top model it allows more options and advantages including lower scope mounting than the standard model.
Yankee Hill Manufacturing A.R.M.S. & GG&G all make good back up iron sights (BUIS)
Zeiss Z Point Eotech and Aimpoint are all good reflex sights
The ACOG TA11 and TA31 are good solid combat scopes if you want to spend that much (they can cost more than the rifle though
) check AR.15.com or e bay for deals on used ones.
A Leupold VXII 1-4X (inexpensive) or 1.5-5X is a good choice as is a Trijicon TR-21 they offer both eyes open for close range fast and furious Zombie shooting but some level of magnification for target identification and engagement at longer ranges than the red dot or reflex sights.
Low power variable scopes offer both close range speed and medium range precision for less $ than an ACOG and about the same $ as an Eotech or Aimpoint with mounts.
A good .22 LR conversion kit allows cheap practice or if the budget is tight a few sub-caliber .22LR chamber inserts go a long way for small critters or plinking and practice.
Spare parts and magazines are a must as is a good case and cleaning gear (one piece dewey rod for example)
And last of all any rifle is a club without ammunition and the AR15 is not that great of a club!