first time building a 7.62x39 upper...i've read the basic FAQ on it, aside from whats listed there...anyone have any "been there done that" wisdom that you'd recommend to doing it right the first time?

i am using the Green Mountain 7.62x39 barrel and either the AIM bolt or the Right to Bear 0.125" bolt completion kit

I have had good luck so far with C-Products magazines.

I just built one recently. I'm happy with the reliability after 108 rounds.
Items used,
1) Sico F.H. for my Spec War
2) KAK 16" melanite barrel
3) Odin works Adjustable gas regulator
4) Seekins NOXS 12" Foreend
5) AIM NiB BCG
6) Misc upper rec. I had in my parts bucket

Obviously I need more time behind it but I'm liking it so far.

I tested it with two different SBR lowers I have. The Noveske has a LaRue MBT2 With a Spikes T2 buffer and standard Carbine spring and the Aero has a Geissele  SSA-E has a H1 with a standard carbine spring. The upper ran  fine with both lowers but I'm surprised how far open I had to move the gas metering screw to get it to run reliably and I had some minor gas to the face when running suppressed .

I've had issues with light primer strikes on Wolf using a std trigger with the lighter yellow springs.

Using a 2 stage trigger like RRA national match and a longer enhanced firing pin will help eliminate that issue. An increased power hammer spring can also be used if needed and the 2 stage trigger helps reduce the trigger pull.  

I went with a LMT bolt for longevity, but they aren't cheap. I just don't like things that break.

Add a Colt gold extractor spring or o-ring. I've had extraction issues with Wolf too.

With brass cases I've had no problems. X39 brass is expensive.

What purpose are building the firearm for... range toy, hunting??

With GM barrel and a stock carbine spring, I highly suggest the use of H2 buffer. I swapped over after about 750rds, it really smoothed out the recoil impulse. Fwiw, I shoot TulAmmo 95% of the time.

I have many thousands of rounds through my build. I used a cason engineering bolt, GM chrome lined barrel, adjustable gas block, heavy hammer spring and xl FP.

I never have failures to fire or any other malfunctions, the rifle runs 100%. It shoots 1.5-2" at 100 yards with golden tiger, though every once in a while you will get a flier with golden tiger that will expand the group out to 3.5 " or so.

I almost forgot one of the most important parts for a reliable 7.62 x39 build , C products magazines.

I thought CProducts went out of business? Isn't ASC run by new owners on the old Cproducts tooling? Haven't shot my 7.62 a lot yet but have had no issues with the ASC's I have.

C product Defense magazines is what you want.  C products and C Products Defense  is 2 different things.  I think C Products is out of business and has been taken over by ASC

You are correct. These are the only mags I could get to run "100%".

My biggest problem was my barrel's gas port was too small (carbine length). It was a YHM 11.5" barrel that I had cut down to 10.5". I had to drill it out and then tune it with an SLR Sentry 7. Gun runs like a top and shoots amazing flat with the recoil of a BB gun.