So far I have:

Black Rain billet lower, Efile Form1 approved.
PSA LPK, milspec buffer tube.
MAGPUL ACS stock.
PSA premium 556 BCG(needs 9mm bolt).
KAK 9mm mag block, https://www.kakindustry.com/ar15-9mm-mag-adaptor
Standard M4 upper receiver.


It's gonna be suppressed with YHM QD flash bidder, 5/8-24 or 1/2-28.

Barrel length probably 4.5 - 9".

Don't care about handguard, for now.

What's the hives recommendations on the rest of the parts?

Buy a dedicated lower

Sorry, but a milspec lower looks goofy and has a bandaid fix via the mag block.
I guess I don't have anything constructive to add expect that everyone who is honest about the AR9 recommends a JP silent capture spring.

I don't know of any 5/8x24 threaded barrels so you're pigeon holed there, maybe a Faxon/Ballistic Advantage 8" barrel and chop to 4" and rethread?

Look at Spinta for a 9mm bolt

Are you going to recess the can under the rail or not? That will play a big factor in rail options

A billet lower isn't milspec.

Any forged lower can be used.  If you don't want a pinned block, get one from RRA.  It is virtually invisible when installed.  

It is obvious that you are overthinking this.  I bought a RRA upper from ADCO which they lightened to 10.5".  I don't know what stamped lower I put it on but it has the block RRA sells and it runs like a swiss watch. My bolt is ramped, I run an ALG enhanced FCG, and a Spikes ST-T2 buffer.  

Sorry, I meant it wasn't a 9mm lower.

It was a pre-41F lower.  I don't want to deal with a dedicated 9mm lower post 41F.

I have accumulated these parts over time, it's more of an after thought than what I have to have.  I bought 2 45 cans last year, but decided stick with 9mm, not adding 45 caliber to my inventory.

As for the barrel thread pitch, I have few 1/2-28 laying around, I can bore it out on a lathe to fit over 9mm muzzle if needed, so that's not a problem.