After 20 years, this is what I know about ARs... (Revision 1.2 on Page 10)
The following is my absolutely free advice to civilian AR owners. I am nothing more than a hobbyist of two decades who has spent north of 100,000 dollars on guns, ammo and training in the last 20 years. This is what I can boil it down to and save you about $97,000.
1. Get a high-quality Mil Spec AR. This seems like a no-brainer. But a lot of folks insist on either (a) buying cheap because it is "just as good" or (b) buying something exotic because they have heard that ARs are crap (usually based on experience from the (a) crowd). My advice: COLT first and Daniel Defense or Noveske Second.
2. Stick with widely adopted optics. If I had all the $$ I have wasted on accessories back I could easily buy a Porsche (used). My advice: Get an Aimpoint M4. They are bomb-proof and run on universally availible AA batteries (for EIGHT YEARS).
3. Don't go crazy with your flashlights. Just get an X300. It will be great for Indoor use and you can use your rifle as a flashlight holster for your pistol.
4. Skip the magnification.
5. Keep your slings simple. I use a Vickers, but a factory sling is pretty damn good (flame suit on).
6. Use Gi mags. They are cheap and work fine. Magpul/HK/ETC mags are fine, but improve nothing but bring other stuff to the table like weight, weird covers and cost. Also, don't buy 200 mags. You need about 10. 15 MAX.
7. Use GI ammo. Availible, relativley cheap, predictable.
8. Just get a 16 inch barrel and keep the birdcage on it unless you have a supressor.
9. Take all the money I have saved you and place (1/2) towards paying off all your other debts and the other (1/2) in checks to Paul Hotaling for Hackathorn or Vickers' classes (or other top-drawer training).
10. Do not stockpile insane amounts of parts or ammo. Keep about 1000 rounds of absolutely sacred untoucable ammo and a little parts kit full of colt parts.