Build your own. It’s not that hard, and you will know exactly what you’re getting.
You can find everything you need here:
http://www.bravocompanyusa.com/
I've built several AR-15 kits, and I think the best source for the basic kit is http://www.model1sales.com/
I bought several AR-15 receivers locally for $99 and got the kits from Model 1. The most expensive kit was the Dissipator at about $550. If you build the kit yourself you will have a much better understanding of how it works and you will have more options that you can choose when you customize it for your personal use.
I've owned an AK-47 for twenty-five years and it has never jammed or not fired. At one time I didn't clean it for about 5 years just to see what it took to make it jam. I finally got so embarrassed from having such a filthy weapon that I broke down and cleaned it.
I bought one of the first Mini-30 rifles and the damned thing stunk up the range. The barrels were crap and never should have been put on ANY decent rifle. God alone knows where half my shots went to. Stunningly inaccurate. I finally sent it off to be rebarreled and had a stronger spring installed so it would fire cheap Wolf ammo. I spent as much money rebarreling and improving the rifle as I did on the initial purchase.
That was the first and last Ruger I'll ever buy.
If you build an AR-15 you will have access to 40 years of modernization and after market improvements. It will be easier to find replacement parts for the AR-15 if anything bad happens.
I hope this helps a bit.
I don’t have much to add. I’d suggest not getting an upper with a fixed carry handle because the options for optics are better with a flat-top(and you can just add a detachable carry handle if you like that look.) An adjustable stock looks cool(and I’m sure has value in fitting different people with widely varying dimensions or when using body armor or not), but I think I’d prefer a fixed stock on my AR15 and probably will change that on mine someday. As far as brands, midrange brands are roughly comparable—CMMG(might be considered the top of the midrange), Rock River, Bushmaster(which I own), DPMS, S&W, Stag, Armalite and I’m sure I’m forgetting a few. Yeah, some are closer to milspec than others, but they’re all decent AR15’s. Colt, LMT, Noveske would be a step up from those. The brands that I hear or read the most negative comments about are Vulcan, Hesse, and to a lesser degree Olympic.
The AR15 is like Legos. You can put together pretty much whatever you want. If you want the most opportunity for customizing, you'll want to start with a flattop receiver to allow for iron sights, optics or both
Next think about barrel length. Do you want a short, easy handling gun, or one with more long range potential. Short means a barrel that has to be 16" (including flash hider), longer means 20" or longer.
Short usually means a collapsible butt stock, longer means a fixed one. Bushmaster is a good name, Rocky River means good value but there are dozens of good makers out there. Spend a few days research and you'll never look back.
Although the AR is one of the most forgiving rifles, as it can be built from scratch using many different mfgs to complete a good rifle, my only suggestion would be use good quality parts.
You can grab a off the shelf Bushmaster DCM for the match grade stuff at around 1K, or you can build your own in various calibers.
The last I built was on a DPMS lower, with a Model 1 Sales CAR A4 heavy barreled Upper chambered in 7.62x39, everyone that has fired the weapon loves it.
Don’t get lost in the thought of putting one together, it is fairly simple, buy a factory head-spaced Upper and you will only need the most basic of tools for the rest.
A friend decided he wanted to put one togther from parts... and that is what he did! He bought all the parts (including the stripped Lower) and we sat at the table and fully put the rifle together in about 30 minutes, afterwords he just looked at the finished product and said “That’s it? I though it would be much more difficult”.
Some good mfgs to start with... DPMS, Rock River, Bushmaster, Stag Arms, Olympic Arms.
Its just a platform for blinging. Buy something like you would buy a precision tool. Some people swear they can earn their daily wages with nothing but Craftsmen tools. I own all Snapon, the same tools I bought 30 years ago are still earning me a paycheck Though I have several AR15's my choice and my favorite is my Springfield Armory M1A. 
pflr
lots of good info here, so the only thing i’ll suggest is get a flat top, and spend some money on quality optics.
used to have one with a carry handle, with a scope on it, and the thing was an absolute tack driver, but it was only good at one distance.
I have three AR's. All .223. One I bought, one I built, and one I won in a drawing. I'd trade two of 'em for a NM M1-A with a 22" barrel and a walnut stock.
The .223 is a good round to about 300 yrds, but I want something that I could possibly tune up to hit out to 800yrds and still have some *umph* left.
Excellent comments, tremendous advice, and please excuse my inexact phrases in my original post. You all gave me plenty to think about, and this turned out to be a fairly decent discussion thread....
Many thanks!
Sabre Defense. Complete rifles or parts. Made in USA (nashville). Suppliers of M-16 rifles to military (one of only 3 manufacturers). Also supplies .50 cal. barrels for the Ma Duce and the .30 cal mini-gun. I own the Sabre light and love it. 799 at my local fun shop and it came with tactical case, 2 30 rd mags, otis cleaning kit and a cheap but functional sling. BTW, they don’t just get parts from other suppliers. They make 90% of their parts in house including their barrels. 100% reliable so far (about 900 rounds). If you have a reliability problem with an AR it’s most likely due to the magazine. Go to AR-15.com for great info on the rifle!
First I think you need to define what kind of “AR” you are looking for. Do you want a full on long barrel or an M-4 style? You mention tactical situations but many consider a long barrel not suitable for that. Do you want a tactical style optics or conventional glass? In short what do you think you are going to do with this weapon when you need it for it’s intended use?
BTW you should be able to put a long AR together for under $800, we do it all the time.
Get yourself a stripped lower and then contact either Del-Ton or Bravo Company for a parts kit. You’ll be able to put a decent AR15 together that way for about $500-$600. Then you can customize as you see fit.
If you have no experience assembling AR lowers, I’m sure there are competent smiths around you that can.
The original stoner design and the majority of ar15s use a direct gas impingement system. While it does pour hot gas back into the chamber it's significantly more accurate then most piston systems at the cost of having to clean your chamber on occasion. This is one of the primary reasons ARs beat the M1A in most service level competitions. AK variants and Ruger minis simply are not in the same ball park when it comes to accuracy.
I would suggest building your own rifle. There is nothing that teaches you the inner workings better then this. The trigger, barrel, and sights/optics are what will determine your accuracy so don't scrimp in these areas.
Determine what you are going to use the rifle for. Determine the best caliber and barrel length, determine if iron sights, red dots, or magnified optics suit that choice, and buy the best trigger you can afford. Make sure the forend you choose is a free float and everything else is matter of personal preference. AR15.com and Brownells both have step by step guides.
When you say “tactical” what do you mean? For short range work, a pistol caliber carbine in .40 S&W or .45 APC (like the Hi-Point) works well. They are inexpensive and reliable.
There is a wide variance in AR15 prices, mainly having to do with their conformance to mil-specs. Mil-spec not only specifies dimensions, but materials, manufacturing processes and testing protocols. Somewhere in my archives is a table showing comformance levels of various products.
Colt rifles are mil-spec, except for their take-down pins. I believe this was done to prevent using M16 lowers with AR15 uppers. Smith and Wesson’s M&P was second on the list.
Rock River, Bushmaster, DPMS all fell somewhere in the middle. Mil-spec requirements are to ensure reliable operation in the most hostile of environments. Unless you are taking your rifle to Afganistan, the Arctic Circle or New Guinea, a middling rifle will probably work for you just fine.
My 2cents.