goto AR15.com and look at what most of the "shooters" are building.
16" for muzzle velocity (you want ~3000 ft/s out to distance that you intend to use). 14.5" is good for pretty much CQB and the loss of 150 ft/s may not seem large, but it'll buy you 100-150 yards of "frangible" power
Mid length gas tube for increased reliability and less problem with gas tube overheating and stoppage due to warping.
Free float tube for accuracy and increased reliability (barrel can bend around the fixed forward point).
I don't have or will likely have a grenade launcher, so go with the regular profile not the M4 profile.
Chrome lined barrel - you may loose a 1/4 MOA on accuracy but will be able to use a larger variety of rounds (corrosive and non corrosive) and probably wear less in the long term due to the lubricity of chrome.
Fixed stock, unless you plan on wearing body armor or really heavy clothing or have multiple people use it. Fixed = less moving parts, solid purchase, better cheek weld... and the AR is for me, the SKS for anyone playing support.
Stay away from all the knick knacks... simplicity and balance is what is critical. My rec is to add on an optic and maybe a fore grip... I find the flash light of limited value unless you're clearing houses or caves. For HD there are much better weapons to mount the flashlight to such as:
Just my $0.02. YMMV, but I had some help and a lot of research.
Cheers,