A couple things I would bring to the OP's attention.
1. Look at barrel life for a stainless barrel vs barrel life from a quality chrome lined barrel (Criterion). You will need to shoot ALOT to become truly adept with these systems. I go weekly and burn through 200-300 rounds in a dynamic shooting meet.
2. Given your concerns think seriously about getting out and shooting ALOT. Ive got several pretty high end builds. For me a suppressor is almost essential. That has me switching from my Vltor Mur upper, ADM UIC, Geissele MK4 handgaurd, and Criterion barrel to a PWS. I never would've known about this had I not started going with my buddy weekly.
3. Given your concerns and aspirations you need to learn to load your own ammo. I was never able to reach 1 moa until I loaded my own. Im not a sharpshooter like some of these guys, but I was on the SWAT team when I worked in LE before I went to Med School.
Regarding weapons.... be prudent and recognize what you dont know. Stockpiling parts is a great idea IMO. Give it time before you build. Personally if I could go back I'd start with a 14.5" with Criterion barrel, and quality upper and lower with Vltor A5 buffer system and Geissele trigger. I prefer ambi lowers. That setup with do 95% of what the others will do.
After all the ARs I built and trained with, my "go-to" is a 13" SCAR 17. Im not as fast with it and its heavier. Im a big muscular guy so Im not all that fast as it is. Ive got a QD Atlas bipod on my gear and a 1-6 Trijicon on it. I can hit just about anything within 600 yards with my hand loads.
For people that can afford the investment I feel like 300blk out is superior in a SBR. Shooting 110grain vmax supers out of my 8" barrel is so much quieter and far more energy than 77 grain from an 11-12" 556. I have my 11.5" barrel squirrelled away somewhere although I doubt I'd ever reach for it. Between those 3 I've got some great options. If I'm going for 1000 then I bring out the 50 cal. Being as my target is 1000+ yards away its no rush. And no concerns about terminal energy.
When I REALLY got into this I had plenty to spend on the best of the best. I had quite a bit of familiarity with firearms before going to the shoot with my buddy. He's a penny pincher and I built him an Aero build with a Faxon gunner barrel and went as... cost effective as I could. I think I even bought the barrel for him. Despite having my perfectly tuned Vltor, Criterion, Geissele, and ADM build he embarrassed me. He was probably 25% faster and just as accurate. Remember that I did those drills as a SWAT member. Build capable weapons, but they will only be as capable as you are. Ammo and training time are what makes a great shooter (in all respects). Even with tons of money to burn, loading your own ammo (especially in our current climate is essential). I purchased maybe 750 API projectiles a couple years ago for less than $1 a bullet. Now no one will even ship them here. Guess who's not worried? Stock pile what you can afford (and extra bolts, springs, detents, etc).Wait and train a bit before you build out several systems. If I had it to do over several of my systems would be piston driven. While the SCAR certainly has its detractors and is unreasonably priced (I dont want to think of what I've got in mine. The 13" barrel alone was $1300, adjustable regulator, KDG handguard, Geissele trigger, etc. But Im comfortable with it whether doing run and gun or prone up to 600 yards... and I'm not the best long range shooter. I spent the money to equip the 16" barrel with a match KDG rail and adjustable regulator. If I HAD to grab one, that system and the 16" barrel would be it.
Sorry for rambling. Ive been in your shoes. Fortunately it sort of worked out. Originally I went with a single stage RCBS press that could handle 50BMG. Loading 9mm, 556, and 300blk out just took way too long so I added a Dillon 750. I still have the RCBS setup to do 50BMG or specific processes, but Im setting up tool heads for the other calibers so I can crank them out in the Dillon. If you're thinking of getting proficient in distance shooting, you'll need to load your own to get the cost/quality in my experience. Otherwise you'll spend years wondering how everyone else is shooting sub MOA and you can't hit MOA.