I'll give you my thoughts....
For Butt Stock
1)Buffer tube (If not included with Stock) A Standard Rifle Length should work.
2) Buffer and buffer spring....I don't shoot the .224, but I'd read a bit.....You might want a heavy buffer in it to help slow down the bolt slam, but if you go adjustable gas block, you could probably tune it down enough.....Both these make a big difference in the 6.5 Grendel.
Lower receiver:
1) I'd buy a standard lower parts kit from a REPUTABLE manufacture, (like Aero) to make sure you have all parts needed, and than upgrade individual parts to your tastes or needs....ie-Bolt release, mag release, safety, ect..... You can buy these minus the trigger group since you have that already.
2) might want an accu wedge.....But check, some of the new Aeros have a bolt that tightens to remove play in the receivers.
Barrel/Upper receiver
1)-Does the JP enhanced bolt come with a bolt Carrier? If not, a bolt carrier is needed..... Again, I have bought a few different brands and as long as its quality brand, I haven't noticed a difference......Young Manufacturing national match bolt carrier is what I put in my last build.....They are heavier, and a snugger fit....I won't say it makes it more accurate, but it's a really nice bolt.
2) Charging handle.... Lots of options from standard to expensive. I have both, and really like my BMC Gunfighter in medium.
3)You will need a Gas block and Gas tube for your barrel, Make sure your one of them comes with the roll pin to connect them together. Check your barrel specs for gas tube length.....It's probably rifle length, but check before purchasing. I'd buy a adjustable gas block.....there is a huge price swing on these, but if you want a good adjustable gas block on a budget, check out Wojtek Weaponry. They are cheap and solid, but are more of a set and forget style rather than a finger adjustable between loads style.
4) Muzzle Threaded? Breaks are a total personal preference!
I hope I didn't forget anything....
. I usually buy a upper and lower parts kit for each build, just to make sure I have all of the little pins an bolts to finish it. I usually only add a ambidextrous safety, and sometimes a extended bolt release, to the standard lower parts kit.
I've put more than a few together, and all of them have shot to what I expected from the barrel quality.......I don't get really hung up on brands for the small pieces I just order from a company that I'm sure is quality Mil-Spec and go from there. I think most of the accuracy from an AR comes from the Barrel, Trigger, and a solid stock, so I think you are most of the way there!
