I think the only truly good reason to do .300 blk is for subsonic builds, but you can't buy a suppressor in CA. If not going suppressed, then 7.62x39 is cheaper to buy, and performs better, IIRC.
Anyway, the options are endless. My advice is that you think long and hard about what you want to do with it. Think about what size bullet (how many grains) you will shoot most. Will you hunt with it? Will you use it for home defense (and I mean, is this the gun you plan to grab in an emergency, not just one you MIGHT grab)? Let your function determine 95% of the form, or you will end up wasting money.
Most guys are going to shoot 55gr FMJ's almost exclusively, and probably just for occasional plinking. In this case, a low-end 1:9 twist upper with a cheapish bolt carrier group (BCG) might work for you. Either an M4-length or Mid-length gas system will work, but unless you go with a 14.7" barrel with a pinned flash hider, the M4 gas system is unnecessarily short and has a sharper impulse (recoil), from what I understand. I would recommend a 16" or longer barrel with midlength gas system. Don't confuse the M4 gas system with the M4 feed ramps. The feed ramps are, in my opinion, a good feature, and don't hurt anything, even when not building an M4-style rifle. If you're going for a super-lightweight build, the M4 gas system and forearm will weigh less.
Some guys intend to do some serious training with their weapons, going to classes/courses, etc. For this purpose, I would recommend a higher-quality build. Many people recommend BCM (Bravo Company) uppers, with 1:7 twist chrome-lined barrel. The 1:7 allows you to shoot longer/heavier bullets.
Pick a stock you like. If you have the opportunity to shoot a buddy's AR, see if you like the stock. Find another buddy that has a different style stock, and shoot his. Forget all the junk about "mil-spec" buffer tubes. I seriously doubt you'll break one.
Trigger and FCG: again, what do you want to do? For plinking, a cheapo Fire Control Group (FCG) will work just fine. Or you can get a Geissle for $$$$$.
If you want a Tacti-Cool build, then I recommend you watch this video, and duplicate their build exactly:
https://www.youtube.com/watch?v=q362H-xg0ZA