I'm going to disagree with most folks here.
Buy a factory built gun.
You get a LIFETIME warranty (SIG, Ruger, Stag), in most cases transferavble to your next of kin.
These days factory built guns are only slightly more money than building a franken-AR on your own.
Also, beware the "Experts" on the internet. They will convince you the rifle will explode in your hands unless you get Hyper-Platinum-nitride-titanium coatings with the cold treatments and ceracoatings... all bullshit.
Akin to getting your car tires filled with Nitrogen.
There's one born every minute...
"Build your Own" - for the ultra-low budget.
Back when I bought my first AR, I took advice of people saying to build it on my own.
Sure, it was the cheapest route... but also resulted in a sub-par AR I wasn't happy with, not to mention it was a multi-week pain in the ass and required spending more on tools, etc.
Then there's always the chance you slip or make a mistake and scratch/ding the rifle in process.
Buying factory means no headaches, and hitting the range right from the gun shop.
"Buy complete Upper/Lower"
My 2nd AR I bought a complete Upper and complete Lower from the same maker.
No hammers, punches, or other crap necessary.
Much nicer build than the franken-AR, but still missing the Warranty and fit/finish of a quality factory made rifle.
#2 best option.
Features to focus on:
- Barrel Twist - Try to go for a 1:7 twist. Can handle heavier bullets for longer distances.
- Barrel Chrome Lined - there are other more exotic coatings now. None of them are necessary.
- "Features" - I'm a lefty, and I know the SIG M400 has Ambidextrous mag release, bolt safety, charging handle and upgraded/enhanced everything else on it. The lower receiver is also reinforced in key areas, and the Lower has a QD mount built-in.
Personally, I like the Magpul furniture. Comfortable and very light weight.
Most of the higher-end AR15 accessories are Mall-Ninja cool, and do nothing other than add weight, and typically serve little to no purpose outside of warfare.
I laugh every time I see an idiot at the range with a $3000 AR15, with lights and grips and lasers hanging off of it, shooting match -grade ammunition at paper targets.
By the way... there are Sig Sauer M400's on sale which come with a Romeo red-dot sight. Good quality sight for the money.
I know of an FFL who has one.
PM if interested.