On my M4's, I've always done:
Carrier Comp Ti mag tube and spring and USA made follower
Geissele hammer
I used to run the CC Ti/A2 charging handle as well, but found it superflous.
I make my own bolt-release using a 1911 4-40 threaded button from Shooter Supply, and a threaded bolt-release from another company. It's very unobtrusive and gets the job done in a classy and functional way.
The C-stock is only for optics/body armor. There are no good stocks for this weapon unless you're 6'2+, as the Urbino attachment system sucks in this application, IMO.
All of my friends who have shot people prefer the 5.56 kind of M4's, and a huge reason is that you can suppress the M4 and won't suffer hearing loss nearly at the same level. This makes sense to me, and I hardly ever shot my M4's of the 12ga variety, so I sold them.
Also, I LOVE the mil-spec barrel. If I buy another M4 (12ga), I'm going to outfit it with a current issue barrel, as well. I just prefer the fixed choke.