I just acquired my first AR last month, so I can relate to the information overload. It's easiest to buy a complete rifle, but you might have a hard time finding one at your budget. You might be able to find a sub $700 AR, but they can get expensive very quickly.
Your next option would be to buy a separate upper and lower (people say that can save you some money by bypassing an excise tax, but...). And you assemble an upper and lower by 2 takedown pins, so it's not a complicated assembly. In fact you'll be repeating half of this procedure most times you clean the rifle.
I'd recommend buying a complete upper because you need special tools to assemble one and since this is your first AR, you probably don't have the experience or tools to do it.
You can buy a complete or stripped lower. You can assemble a stripped lower & parts kit with a small punch and "normal" tools. And there are videos on Brownells, youtube, and elsewhere to show you how to do it.
The lower receiver is considered the firearm portion and is the only thing you have to purchase via FFL. You can buy upper parts or a reciever from anyone and have them shipped directly to you.
Also, Lower receivers from most manufacturers you will come across are fine. Daniel Defense (DD), Bravo Company USA (BCM) and Lewis Machine Tool (LMT) are some of the higher end parts makers, but lowers from CMMG, Panther (DPMS), Stag, MegaGator, Spikes, Rock River Arms (RRA) and others are fine.
Some companies have questionable manufacturing practices in their upper assemblies. Sometimes they don't include "proper" feed ramps, neglect to stake the bolts that hold the gas key onto the bolt carrier, which can allow the gas key to come loose, and other issues which don't meet the military specs and can lead to a less reliable (and accurate?) rifle.
Also, you must have a 16" barrel, including any permanently mounted extensions, to meet NFA requirements. If your barrel is < 16" it's considered a Short Barreled Rifle (SBR) and needs a stamp. This is why you'll find 14.5" barrels with pinned muzzle brakes or flash hiders. Since you are planning on shooting 200-300 yds, an 18" or 20" barrel is probably more appropriate for you, but you probably know that.
You might want to check out m4carbine.net. It's an AR forum and they drink their own kool aid sometimes, but they do have some useful knowledge threads. You can also check out tacticalyellowvisor.net. It has a comparison chart that compares the major AR brands and explains all the typical features found in an AR. It's under Charts/M4 Chart and it has a link to it in google document form. (I'd provide links, but I'm not sure if that's against the TOS?)