While I'me quite familiar woth the AR15, the other AR's are a mystery to me. I decided I want a .308 AR, so it's happening. I picked up a stripped Aero M5 and upper yesterdqay. I dont plan on a fast build. I figure one part/group a month.
I plan on it being 18" or so. It will be threaded for QD brake.
So where should I start?

I'm thinking butt stock, LPK, pistol grip

For the upper, charging handle, bolt, barrel (which may go to ADCO for threading if needed), free float tube.

On the barrels, i recall reading something about cut outs in the barrel extension similar to the M4 feedramps.  what's that all about?

What do you foresee this rifles purpose to be?

Mostly range time. Steels plates out  to 400. Deer and pig hunting as well.

If you're already committed to the Aero M5 receiver set, get a 16" or 18" barrel from a decent source, and balance out the furniture to fit your frame/build/reach.

It will be plenty for pigs and 400yds on steel.

Barrel sources:

Fulton Armory
Brownell's
Ranier Arms

Here's your stock LUTH-AR MBA Stock  I have one of these and it is badass, it's similar in function to a Magpul PRS but a lot lighter and cheaper.

Also I'm a big fan of this barrel Mega 18" barrel  it's stainless and nitrided and guaranteed sub-MOA with match ammo.  You can get it from there for $284 shipped with some codes.  I have the 16" and it groups very nice.

100 yards with reman match ammo

I am commited to the M5 set, as it's sitting in the safe already and I got a pretty good deal.

I like the look of that stock, I'll have to check it out. Looks like it used a full length buffer tube, which I like.

On the barrel, will 2" make any real difference between 16 and 18". If not, I'd go for the 16, as it will almost always have a can on it. Speaking of which, are adjustable gas blocks made for the barrel linked?

I'd go for 16 or even shorter with a pinned qd mount. 400 yards is chump change for a good barrel. And the short and lighter the easier it is to maneuver it, obviously.
I'm a fan of the UBR for 308's, the weight brings the balance rearward some which in my LMT is pretty far forward, may not be an issue with your set.

It just takes a standard .750 gas block, so any block with .750 diameter will fit.  I went with 16" over 18" since I will be suppressing it and want to keep the weight as close as possible to the gun.