@RockAndSage those Vseven rifles look awesome but a little out of price range, budget is around 3k including optic, can probably build one around 6lb a little cheaper + optic weight.
@chicoredneck @mother_plucker for mags is was planning on getting a couple duramags
https://dura-mag.com/duramag-ss-6-5-grendel-6mm-arc/
@Old-Cat awesome links, definitely lapping/bedding the receiver on this AR build.
Trying to find a Faxon Match Series Gunner 6mm ARC in 18" or might just do a proof carbon fiber 18" but they are a little pricey.
You can build a killer gun for $3k, I think you're in good shape.
If I were in your position, I'd prioritize the parts of the build, decide what are non-negotiables and what doesn't matter as much, and go from there.
For me, non-negotiables are top-grade bolt, carrier, barrel, and trigger. You likely know a bunch of what I'll share below, but there might be some nuggets in here you or others might find helpful:
Bolt Carriers - The BCG is the heart of reliability (or death of it) in an AR, so it needs to come from a company that gauges every single part, properly stakes the gas key, and uses proper fasteners on the gas key. Forward Controls Design, Sons of Liberty Gun Works, and Modern Armory have pristine quality control on these points.
Bolt - For a 6 ARC, bolt reliability is even more important than in standard ARs, as the cartridge head size requires the bolt to have less material around the bolt face than a .223/5.56 bolt. They're known for shearing lugs and breaking extractors because of this. The best quality 6 ARC bolts I know of come from JP Rifles. It would be an extremely good idea to keep a spare extractor on hand as well.
Barrels - For barrels, your mentioning of Faxon and Proof are solid choices. I own a Proof-barreled ultralight AR, and love it. Keep in mind though that carbon-fiber barrels aren't necessarily lighter than a lightweight-profiled standard barrel, they're just lighter than a barrel of equal diameter. The advantage you get with them is that they're superbly stiff, and that's what I wanted for an ultralight build that I'd have a suppressor on (same thing if I ran a comp). Carbon barrels also seem to be more forgiving with handloading and finding a factory load that will shoot great...something about the construction seems to make the accuracy nodes bigger and easier to find.
Triggers - For triggers, this is much more personal choice, but having one that's reliable with an extremely crisp break is non-negotiable for me. One thing to know is that in the extremes, an open-design trigger is notably more reliable than a cassette style, as the cassette triggers seem to have a much harder time working when things like excess fouling from suppressors, popped primers, or grit get into them. As long as you know those limitations, they're great. I'd have zero hesitation running one in a hunting gun. For cassettes, I'm hearing extremely positive reviews of the Blackout Defense triggers, and for open designs, it's hard to beat Geissele, but I do hear LaRues can be a good value too.
Adjustable Gas Blocks/Gas Regulation Systems - Thumbs-up as well on running an adjustable gas block, especially for a build and in a somewhat uncommon caliber, if you want to tune for maximum reliability in cycling. Just know that if you expect to be able to tune it throughout the lifetime of the gun (different loads, etc), you need to regularly lubricate and rotate the screws with a penetrating oil in the adjustment mechanism, as they will seize up from fouling over time if you do not. The adjustable blocks from SLR Rifle Works are great choices. Up until recently I'd also have recommended you take a look at the adjustable/gas-regulated BCGs from 2A Armament, but they just got purchased by NEMO Arms, and their adjustable BCGs have disappeared from the market. I own two of these and I frankly wouldn't want to have to go without them. From what I can tell from the 2A/Nemo website they do expect to release a new/upgraded version of the regulated BCG, but for now, the adjustable 2A BCGs are almost impossible to find.
Mags - They can be so hit or miss with non-standard calibers, especially for builds. My best recommendation would be to buy 2 of each brand or capacity you're interested in, to the extent you can budget for it, and just see which ones run best. The ones guys mentioned above have good reputations, but also keep in mind that mags are consumable items, so plan on having spares. More often than not, they'll look just fine externally, but just start not functioning well at some point. Always number/mark your mags for this reason, so that you can ID and swap out a troublesome one quickly.
Everything else for me is a matter of preference, budget priority, and lately, availability. Depending on how high of a priority accuracy is for you, very stiff handguards and tight-tolerance uppers and lowers can be very helpful. Don't go cheap on those, but you'll get fine performance out of a lot of offerings.