What is your budget?
If you are going all out then I would go as followed, BCM 16" carbine or middy depending on what you want, I prefer the middy. throw on a 10" Larue, or Daniel Defense rail, MBUS sights or Troy. One of Magpul's awesome stocks, I personally like the CTR the best. Magpul MIAD grip and a Larue FUG. Then for muzzle device, all out Triple Tap for compensated I also like the PWS and for flash hiders I like Vortex. Followed with some kind of tac-lite (I like Surfire E2 series) and then an optic of your choice, for a build like this I would opt for a T1. YMMV
As far as getting what you want, hit the site vendors Bravo, Larue, and PK. are my personal favorites. My best piece of advise is buy once cry once.
ETA: if you want to build the upper Noveske makes a top notch barrel, and BCM has a new CHF Barrel that is also suppose to be top notch.