I'm not an AR expert, but I've learned a few things with mine.
Setting your sights for one under $1,000 is definitely doable. But I advise looking around at different ones judging them by how they're made, not who made them. So here are somethings to consider. I wish I'd known these before I bought mine, but fortunately, I kind of lucked out.
Besides looking at fully assembled rifles, look into fully assembled uppers and lowers. AR-15's aren't like 1911's, where every part is custom fit. Most parts are interchangeable. You might find it cheaper to buy fully assembled upper/lower than a complete rifle.
Also, it would do you well to sit down and write out what you want from this rifle. Hunting? Plinking? Target shooting? Self/Home defense? Or they're so cool, you've just got to have one? Any of these are good reasons, but might change what you're looking for.
I'll cover some basics. Rail. Look for a flat top and quadrail. The old M16 handle isn't very useful and most people get rid of them. With a flat top and quad rail, you can dress up your Barbie gun any way you want.
Sights. BUIS are what I recommend. That stands for Back Up Iron Sights. Meaning, they flip down out of the way when you don't want to use them.
Barrel. What length do you want? I think the most popular barrels are just over 18" which is carbine length. You can get shorter or longer, but this is a good all around length for most of us. If you need one a different length, ask yourself why. Shorter ones are more compact. Longer ones are... well, longer. They don't really add accuracy or velocity out beyond 18". Do you need it chrome lined? Probably not, unless you live where it's really humid and/or you don't plan on cleaning your rifle properly. Chrome barrels are not more accurate than plain barrels. In fact, defects in the plating process might actually negatively impact your rifle's performance. Twist. Most 18" AR's are 1:9 twist. Some are 1:7 and some may be higher than 1:9. The lower the 2nd number, the faster the bullet spins when fired. The subject of twist can get really involved. Just know that 1:9 is a good twist for most uses.
Charging System. Most AR's use a DGI system (Direct Gas Impingement), however some use a pistol/rod/tappet system. Ruger's use this piston system. DGI systems are slightly lighter and possibly more accurate and longer lasting. Piston systems run cleaner and don't get the chamber area nearly as hot. Getting an AR too hot, can cause a round to cook-off, or fire due to heat. Unless you're rapidly firing 2 or 3 full mags without resting in between, this isn't likely to happen. But it's less likely to happen with a piston system. DGI systems advantage is that they exert straight back pressure on the bolt carrier group (BCG). Piston systems on AR's create slightly angled force on the BCG, which may impact accuracy and service life. But DGI systems blow the dirt and crud from the barrel back into the chamber/BCG area. Because of this, many AR users run them "wet", meaning they really oil them up to keep them working - again, another subject.
BCG - I highly recommend a Nickel Boron plated bolt carrier group. This is a hard, slick coating that really extends service life and reliability. You can easily buy a rifle without one and replace it later, if you choose. They're essentially a drop-in part. I added mine. I ordered it from Brownell's and it's made by Spike's Tactical. Staked gas key. Make sure your gas key (an L shaped tube that goes into the bolt area and the other end goes into the gas tube) has two bolts on it. They're subject to violent shaking when the rifle fires and will most likely become loose if they are not staked properly.
Lower. Most lowers work fine as is. Some have do-dad's that you can easily install yourself, such as ambi-selectors, etc. Your basic GI trigger, IMO, is not a bad trigger. Of course, you can always buy a better one and usually those are drop-in parts, too.
Charging handle. Most guys will replace their charging handle, since the GI version doesn't often give you a very good grip. I've put a Bravo Company charging handle in mine. YMMV
Optics: Way too much to cover in this post. Again, consider your purpose of use. Hunting? Maybe you want a scope. But most of us prefer a red dot system. Red dots allow instant target acquisition and show you where your round will hit. They also allow you to shoot with both eyes open much easier than a scope. The rule of thumb here, is that good optics aren't cheap and cheap optics aren't good. I've got an EOTech MPO III on my AR, which comes with a 3x magnifier. This magnifier will flip up/down so that you can easily use your red dot with or without it.
Co-witness. Co-witness is when you can see and use your iron sights with and through your optic. It's good to have, but not mandatory. I have flip up/down BUIS on mine and they co-witness with my red dot setup.
There is a set of 4 books written by Patrick Sweeney about AR-15's. They're available on Amazon.com. These are highly recommended. Here's a link to the first one.
Amazon.com: Gunsmithing - The AR-15 (9781440208997): Patrick Sweeney: Books
Good luck and have fun. Sometimes searching and researching is half the fun of owning any firearm.
Oh, and one last thing. Don't buy a lot of one type of magazine before you verify your rifle runs well with them. Every gun is different. Most guys prefer the MagPul Pmags, but there are other makers that make good mags, too.