I wanted to add the following:
Regarding the SureFire X300, get the older model (170 lumens) if you plan to use the carbine as a home defense weapon in your home (as opposed to in and around your property in a rural area). Blind yourself a few times with the older lower lumen model by shining it at a wall to prove to yourself that the new 500 lumen model is not something you want to light off indoors at night.
Regarding the optics, you won't need any magnification indoors. If you have a house large enough to need magnification, then you can afford professional armed guards who can handle home defense for you.
There is no human that I am aware of that can line up three points at different distances from his or her eye faster or easier than two points. That's why you need a red dot optic. I prefer Aimpoints to EoTechs for battery life and the ability to leave the sight turned on for years, but it's a personal preference. I would not purchase any EoTech model that does not use a transverse battery mount, like the XPS models do. I own both and use both.
Any accessory that is screwed onto an AR-15 requires blue locktite. If your optic mount has screws, they need locktite. If you have a folding rear sight, it needs locktite. If you shoot your AR-15 regularly you will find anything you didn't locktite on the ground.
I prefer the GG&G and Larue Tactical mounts for optics, but there are a variety of manufacturers out there that make good mounts. In the past, I used ARMS.
The Colt 6920 and 6940 that others have suggested are nice guns, but I can't think of any reason why a civilian would require anything more than a white light and red dot optic. The extra weight in the barrel and handguards make both the 6920 and 6940 carbines noticeably more nose heavy than a 6520 or 6720.
The 6920 and 6940 have the ability to mount grenade launchers because they have government profile barrels. If you need a grenade launcher for home defense, you're gonna need something more powerful than a carbine. The 6940 has a monolithic upper rail for mounting things like magnifiers behind Aimpoints or EoTechs, lasers, night vision devices, and various fore grips and light switches. If you are a SEAL Team 6 or CIA ninja assassin that stuff might be necessary, but I think it's unnecessary for most civilian uses and makes the carbine unecessarily heavy and slower to swing onto target.
The only ninja accessory I can recommend would be a miniature suppressor like the SureFire Micro. Other manufacturers make miniature suppressors, too. Some are quick detach like the SureFire and some thread onto the muzzle. The full size suppressors are too long for indoor use and too heavy. From having fired weapons indoors with no hearing protection, I can relay that this is not an experience you want to have. You will have some minor hearing loss and you will be deaf immediately after you fire the first shot. If you have the time and presence of mind to don hearing protection while someone is busy breaking into your house at 3 AM, I suppose that isn't a problem.
Apart from that, the Colt carbines only meet military quality standards and are not special in any other way. If you can part with more money, you can get a better carbine.
As others have stated, without a good supply of ammunition and quality magazines the AR-15 is a really expensive paper weight and poorly designed club.