Currently own an IWI Zion 16" carbine. Reliable, as accurate as I am, installed ambidextrous controls to make it more leftie-friendly, still running factory trigger, pretty satisfied with the gun as-is.

HOWEVER. During heavy training, after a few hundred concentrated rounds within a short period of time, I'm not crazy about the blowback and powder residue all over my face. I have left-side ejecting rifles in other calibers that solve this problem and would like the same in 5.56.

I already own a Tavor 7 in 7.62, so an X95 would be a logical choice since I am already familiar with the platform. I like the big bullpup but I'm more comfortable with the AR overall.

Therefore, I'm leaning toward building a dedicated, left-side ejecting AR15 instead. I've broken down the Zion but I've never built an AR from parts. One reason I rejected the AR platform for years was simple option-fatigue, as there are just so many choices available that I didn't know the good from bad, the quality stuff from the junk. 

I should also note that I would like to build a true rifle length firearm. I trained CQB with guys running 20 inch barrels and it didn't look as awkward as I expected it to. I shoot varieties of 55 grain almost exclusively and the practical range of this build in the jungle of Appalachia will rarely exceed 100 yards. Still, I would like the ballistic advantages of the longer barrel.

I thought I would start with Stag Arms upper/lower receivers, but from there I really don't know what I'm doing. I don't need the absolute best money can buy, but I want reliable/durable stuff all the same.

Any and all suggestions for a good, mid-range build to these specifications are appreciated.

If going with a Stag lower, their lefty ejecting upper would seem to be an obvious choice.

CMT-TAC

Built 3 lefties with this upper and CMT’s ambi lower.  One a dedicated 22LR.  All ran just fine after assembly.

Not much to it, you slap the upper on the lower and you are done, or just buy the complete rifle.

Now if you want a ground up build, you need a Stag LH complete upper receiver and their LH complete Bolt/Carrier. The rest is standard parts.

There is no point for left handed rifles except for bullpups...
Even army and law enforcement just get you going with the standard setup and your left hand - it truly isn't slowing you down or holding you back.
in fact all the right side charging handle guns like AK, SKS, FAL ... allow you to do mag changes without breaking the cheek weld.
Hks with the charging handle on the left side are a little awkward but nothing that can't be learned.

One hint related to barrel size and chambering:
I have some options in all price ranges and lengths from various manufacturers 9.5 11.75 13.9 14.5 16 18 20...
My most accurate and softest shooting rifle is running an Ballistic Advantage 18" in .223 WYLDE
This is with a rifle length gas system and a rifle length buffer assembly.

…and there it is, the ever so helpful “you don’t know what you need” reply seen in every left hand AR thread.  Wondered when we’d see it pop up.

Speaking for my "people" but you're incorrect.

Unless your argument were to be about logistics. But it wasn't, you used the appeals to mil and police authority to justify your argument.

Our mil for a long time had no solution until the introduction of the shell deflector for left handed shooters which was one of the requirements for the A1's replacement. The AR15 didn't really take off into police use as a replacement to the patrol shotgun until the long after the fact that shell deflectors were now on all rifles from the major manufacturers, so that side too is now tossed out.

Getting an ambidextrous CH makes it easier to use, the claw is not conducive for it's effective use from the left side. In fact, most modern offerings these days have ambidextrous features built in because the this is what a certain portion of their marketed audience wants. You may not, but it doesn't mean that others feel the same as you.

Oh, and one more fun fact but as a former owner of pistol gassed 300 BO 8.5 " barrel, the brass in a right hand configuration still smacks you in the face shooting from the left despite the other fact that the shell deflector is still there. Even Google shows that this isn't an isolated incident either. But it doesn't with a LH upper and LH BCG

Speaking as a lefty…. Ambi safety and charging handle on a standard rifle is all I need.

If I buy complete/assembled upper and lower receivers, yes, but that comes with limitations that may not best fit my needs. Stag only offers its 20" barrel uppers in "classic" configurations, for example, with barrel mounted sights, a fixed/plastic hand guard, etc. This looks cool but it's not really what I'm after.

I'm sure I could assemble a quality, complete rifle from 100% Stag parts, but that's why I've posted here. If I can do better by mixing and matching some stuff, I'd like to know how, or at least be pointed in the right direction. Perhaps I can start with stripped or partial Stag components and fill them in with better stuff..?

The only parts that I would say to go with Stag on are the upper and lower receivers.  The barrel, BCG, handguard, FCG, buffer assembly, etc, etc, etc are wide open.  That is the beuty of the AR-15, there is a universal spec and you can intermix brands with confidence.  I beliefe Stag also offers a lefty bolt so that might be a good choice as well.

It's not too hard to build an AR from scratch overall. The 3 "big" things are barrel nut torque, castle nut torque, and headspacing of the bolt. Which, headspacing is not too big of a concern nowadays for bolts and barrels from reputable manufacturers, and a quick check by a gunsmith to verify.

If I were you, I'd start with a complete lower, an "assembled" upper, and lefty BCG from Stag. Lower can be Stag ambi or whatever other ambi lower you want. Then pick your preferred barrel and hand guard, along with gas block and tube, and just have a gunsmith put that on the upper for you. It would be like 15 minutes or less for the gunsmith to do that. Or, if you have a torque wrench, grab an armorer's wrench with an attachment point for the torque wrench, put the barrel/hand guard on yourself, and just have a gunsmith check headspace quick for you. Even the lower wouldn't be too bad to build from a stripped one, again just need a toque wrench and an armorer's wrench that has an attachment point for the torque wrench for the castle nut.

Everything is a trade off. Always.

I have both standard and left-eject ARs. Left-eject uppers have their place. I don’t need/use them always, but for certain things they are helpful. The only downside is training artifacts when you switch between left/right eject weapons. But for ejection ports that’s very minor.

OP, check out Red-X in addition to Stag. They have complete LH uppers, as well as individual components if you want to build.

As for lowers, I don’t use any left/ambi stuff except selectors. Bolt stops and mag releases are perfectly usable in standard configuration. And as soon as you start adding ambi controls the training artifacts immediately rear their head when you pick up a weapon that doesn’t have them.

Left-eject benefits:
No gas in face. Very big deal if using a can or shooting high rates of fire.
No shell deflector drilling a hole in your chest while slung (not a big deal for a range trip but big deal when carrying all day)
Ejection port is more easily accessible with dominant hand especially if using an optic.

you just need the Stag LH stripped upper, and a LH BCG.  as others have said, the rest is standard parts.  I got my LH BCG's from Cryptic Coatings, they were cheaper at the time.  works just fine on a run-of-the-mill USGI mil-spec lower.

RRA also has LH rifles, and stripped LH uppers & lowers, but if you go with just their stripped LH upper, you also have to get their LH lower, since their LH upper is a little unique, they wanted to maintain the AR-15 appearance with the ejection port cover hinge rod at the bottom, instead of Stag's version at the top.  Won't work with a USGI mil-spec lower, since the bolt catch occupies the same real-estate.

It wasn't my intention to come across as "that guy" telling others that they don't need something.  All of my ARs have raptor ambi charging handles - that's the one thing that really makes a huge difference.
I usually shoot 2-3 times a week and every single time I switch shoulders for a mag or two. I can run guns with optics without any troubles on either side and for combat distances it doesn't make a hell of difference.
If I shoot iron sights, longer distances, prone or from a bench I definitively prefer left eye / left hand. Never hurts to try out new things, learn to be ambidextrous and continuously improve skills.

I'm left handed.

The fact you need proprietary upper and BCG has prevented me building a dedicated left handed upper. Seems like Stag is the go to for these.

I shoot suppressed 100% of the time, and a properly gassed barrel (or other gas limiting features. I don't like AGBs but have used a gemtech SBC on my BCM upper). Criterion mid length is gassed great and my older 13.7 BA hanson was gassed great too.

Plus a standard upper allows for easier/faster malfunction checking since you are facing the ejection port already.

I put G charging handles, and FCD goodies on my lowers (safety, mag release and bolt catch). The only thing it doesn't have is right side bolt catch/relase.

Or I've also got a gucci ADM ambi receiver set which is very nice (and had the right side bolt catch/release built in).

Now all your parts are still interchangeable.

This, I’ve tried a friends left handed Stag and it’s a why after shooting. I am 100% lefty and honestly everything other then the safety and charging handle is a waste of money.