I will be picking up a Colt Registered Receiver (M16 style, not Sporter SP1) around February/March and am sourcing parts for it. It is currently setup in the factory rifle configuration with a fixed stock but I'm going carbine length for a number or reasons. My uppers all have adjustable gas blocks and I'm looking to set the receiver up as a reliable and durable shooter. How would you or how did you spec out yours? Thanks for the help.

NEED TO DETERMINE:
Buffer: JP Silent Captured Spring or use a traditional buffer/spring combination?   If JP SCS, standard weight H1 or the H2 version - I'm not sure if having adjustable gas uppers still need an H2 in auto?

Trigger: Keep factory or replace/upgrade?

Trigger Pins: Use standard pins or anti-rotation pins?

Selector: Keep factory or use some ambi style?

Small Parts (Mag Catch, Takedown Pins, Bolt Release, Buffer Retainer): Leave originals or replace? Of course there won't be a 'performance' benefit but would there be a reliability, durability, or wear benefit?

ALREADY PURCHASED:
Buffer Tube: PWS Enhanced with Ratchet Castle Nut
Grip: Magpul MOE K2+
Trigger Guard: Keeping factory.

I would do:
Geiselle SSF trigger
Vltor A5 or equivalent buffer tube: http://c3junkie.com/?page_id=571
556 Tubb Flat Wire spring (works in both carbine and rifle length buffer tubes)
9mm Blitzkrieg/Kynshot buffer.  Compressed, this buffer is the same length as an A5 buffer.
I'm working on webpage with a lot of full auto tuning details.  
For now, this link goes over the 9mm Guard tuning but the buffer/tube and spring selection is discussed here that I also using in 556.
http://c3junkie.com/?page_id=538

I am also using the PWS Ratcheting castle nut.

I'm using ambi selector but that is all preference.

Which ambi selector and are you running standard pins with the SSF?

Another advantage of the PWS ratcheting castle nut is the ability to (relatively) easily switch between say A5 buffer tube & MVB ARC (pdw stock).

I've got some Bushmaster Ambi selectors I'm using now as well as some KAC ones I got cheap.
I am a C2 so have transferrable and post samples.

I've posted long discussions regarding the trigger/hammer pins.  Yes, I do use them with some caveats.  Read towards the bottom of this link for details: https://www.ar15.com/forums/Armory/is-a-Geissele-SSF-trigger-Worth-it-/23-493199/

First, CONGRATULATIONS.

Buffer: Personally I would go A5 with appropriate A5 buffer for your application. Skip the silent capture. Go with what’s proven.

Trigger: geissele SSF no contest.

Fcg pins: geissele. I used to use kns anti rotation pins, until I removed from one of my m16 lowers and noticed the anodizing was slightly burnished (a tiny bit shiny) where the external pin boss was abutting the receiver. It’s barely visible, but it freaked me out and I no longer use them. Geissele also does not recommend kns pins for their triggers.

Selector: personal preference. Lmt ambi if you’re cool with a traditional length/profile lever on the right side, KAC if you want a scalloped right selector, BAD if you just want to spent money (I like them and have them on a semi AR, I just don’t think they are personally worth the price of admission for me personally).

Small parts: I don’t see any benefit to replacing these. You can also potentially do more harm than good if you crack off one of the bolt catch or trigger guard “ears” that support the respective axis pins.

I second most of this.  I went with the A5 tube (Magpul 10 hole version), Geissele SSF/pins and KAC selector.  I replaced all of the small parts (springs, pins, etc.) with new Colt parts from Brownell's.  
I also replaced the trigger guard with a KAC.  If you want to replace the trigger guard,  use one of these to push out the roll pin and install the new one, instead of doing it the wrong way with a hammer and punch:

https://www.littlecrowgunworks.com/store1/RPP-Roll-Pin-Pusher-p106089763

Takes the worry out of it.

Oh, and in my case, the BAD selector did not work.  I guess they do for some receivers, but not mine.  The back of the selector switch contacted the receiver so I sent it back and got the KAC.  I'm not taking a chance on a selector lever messing up the finish on my M16A2.

I  have  owned a Colt M16 for 22 years. I run it with a 10 or 14.5" upper. I use a KAC 2 stage trigger just because I like it on semi auto.I  did not have a problem with the stock trigger on auto. I use a Colt carbine stock with a H2 buffer. I have heard the A5 buffer smoothes out the the action. I recommend you try the basics before spend a lot of money.

This is sound advice, though I will say that if you’re swapping stocks the a5 is worth doing. Also the geissele SSF is a must have.

Thanks for the replies and advice so far! Sounds like the SSF is the go to trigger. I asked about an ambi safety so that I can see the selector position from either side of the rifle while I am a spectator to someone else shooting it.

In regards to the A5, which I do like the idea of smoother or less chance of bolt bounce, what buffer and springs are being used? I did take note of the Kynshot recommendation but would like to hear of other specific combinations.

The A5 uses a rifle length spring and a dedicated A5 buffer that is available in multiple weights. I am still using a carbine receiver extension, standard carbine spring, and H3 buffer because mine runs great with this combination. If I was starting today, I would use the A5.

Check out a lot of data I put here: http://c3junkie.com/?page_id=654

The great thing about the A5 is using the rifle length spring....The 556 Tubb flat spring works in both a carbine and rifle tube...so it obviously works in the A5 as well.
I go over this testing full auto, suppressed and unsuppressed in the link above.

In conclusion, I would suggest to just get the Tubb flat wire spring for use in your carbine tube if you don't want to switch to the A5 yet.

I mostly agree.

I would go with h2 or h3. I used h1 for many years and swapped to h3 a few years ago and it definitely helps with ROF especially when shooting suppressed.

By regular trigger pins I assume you mean the Geissele pins. I agree.

MOE K2 is my fave for shorties.

KAC ambi selector is scalloped on the right side, making it much more comfortable. I think the way the MOE K2 positions your hand is also helpful to prevent “selector bite”.

Why is everyone so hot on the PWS ratchet receiver extension? Seems a solution in search of a problem.

I'm just using the ratcheting castle nut end endplate not the extension since I'm using A5 system.  Doesn't come loose and no staking.  
Not a big deal if I change my mind on swapping out the buffer tube for some reason.

Interesting. I stand corrected. That is pretty novel. What happens if the ratchet notch on the nut isn’t quite perfectly aligned with the ratchet pawl on the endplate for full torque on the nut? Sort of like how you can’t typically get a aac suppressor fully tight on a 51T mount because fully tight is in between ratchet positions.

I haven't had that issue.  I think because there are so many notches it isn't an issue at least for me and the 3 or 4 builds I'm using these on.

Pretty cool.

Never had a castle nut come loose though. Feel like a drop of Loctite pretty much does the same thing but when youre already spending $20-30k on a lower why not go crazy with a fancy ratcheting castle nut. I replaced everything on my upper and lower with V7 and similar fancy lightweight parts just because I can, I mean $50+ for some high tech alloy version of a steel part is silly but so is owning a firearm that costs 3x as much as the car I drive to the range in.

I would go with Factory Colt for parts, using an H2/H3 buffer depending upon your upper. I use the DPMS ambi selector.

I'd like the ambi selector to have a shorter selector on the right side. Looks like the LMT has full length selectors on both sides, KAC is scalloped (and I don't see it on KACs website), and Colt may be the winner with what looks like a half lever on the right side. I'm not sure how the BAD selector can mar the receiver finish but I'd like to avoid that.

I have a BAD ambi selector on my Colt M16.  Levers do not touch receiver sides at all.  Looks like a good .030 clearance.   I really like the BAD selector,  very smooth and positive stops.

kns makes new "g" pins for use that have the tight .1555 spec the Geissele triggers require so you dont have to use .154 pins.  These were released a long time ago now.