generally stick to the 223/5.56 that is what the gun was built around and it really does work quite well , normally very available and at a reasonable price.
unless you have a special use that would require a different caliber like you are a strait wall cartridge area and intend to hunt with a strait wall cartridge from the AR.
when you start getting away from 223/5.56 you start running into non standardized components not a big issue but something to realize.
an AR15 in 223/5.56 basically everything is made to "mil-Spec" this means a standardized set of blue prints the critical measurements of 2 components from different companies will be withing the allowances of the specification drawings.
Ruger , S&W , PSA , have fairly budget rifles , very functional
Windam , Daniel defense , SIG , Rock River , are a step up
there are a lot more manufacturers out there most all of them make a rifle that goes bang reliably and will hold 2 inch or better groups at 100 yards.
I prefer a free floated hand guard, this like a free floated hunting rifle the barrel does not have contact with the fore-arm/ Hand Guard as it it called on the AR rifles. this means your hand pressure isn't pushing on the barrel but back at the receiver and the barrel can move freely this improves accuracy.
accessories mount generally with the 1913 type rail , M-lok or Key-Mod most everything is now standardizing on M-lok
accessories to have , generally some sort of optic , a red dot or low power optic on carbine length rifles
gas systems there are different lenght gas systems I mostly go with Mid lenght.
pistol , carbine , mid lenght , rifle are the options
as the name suggests pistol is the shortest and is for very short barrels or non 223/5.56 rounds with less gas pressure
Carbine may be found on 10.5 ,11.5 13,5 , 14.5 and some 16 inch barrels this is the M4 carbine lenght used on the military M16 -M4 carbines with 14.5 inch barrels .
Rifle is found on some 18 inch but promarily on 20 inch barrels "rifle" barrels
Mid-Lenght is a compramise between rifle and carbine and is in the middle it provides reliable cycling with 16 and 18 inch barrels while not being overgassed flinging the brass with excess force that carbine can the military errors on the side of more gas is better to extract cases in dirty combat situations.
Mid length in a 16 inch barrel with a clean rifle is easier on the brass , less abrupt movement and generally can lead to a slightly smaller group size as the components of the rifle wait maybe a few thousands of a second longer to start into motion. the cycling feels smoother , many people may not even notice.
from a basic Mil spec parts kit the places to upgrade are most often in the trigger , the 5-6 pound military trigger
inthe same way you might make a bolt action rifle easier to shoot , glass bedding the action int he stock , free floating the barrel and havig the trigger worked to be smooth , with a lower pull weight and a crisp break with no over travel , these things apply to AR rifles as well.
this isn't a time of good prices or good avalability
my preference is a good
-sling with easy to release mounts either QD (quick disconnect) or Para Clips these are locking clips
-an optic , although I have irons sights as well
- a Magpul or other grip that I find comfortable in my hand the original AR grip has a ridge that doesn't fit right in my hand and is a rather small grip for my xxl hands
- improved trigger like a ALG-ACT or a Larue tactical or PSA-EPT these are all basically hardened and or coated with a hardened material that makes them smoother and more crisp in the break
-depending on the use a light mounted very near the muzzle identifying the target is important.
- sometimes a muzzle brake (muzzle brakes make a very flat recoil cycle also reducing the already minimal recoil your sights hardly even move off the target when the rifle is fired)
if you asked last year this time 600-700 would buy you a nice basic complete rifle
today the world troubled and supply is low. here is a gun that was selling for 650ish last year https://palmettostatearmory.com/rug...ee-float-handguard-rifle-black-8529.html
still a good choice in first AR as a complete rifle but more expensive.
building may save you some money especially if you know someone who could help you and has the few tools you will want. I say want because I recently assembled one at he dining room table with about 4 punches a hammer , lube , painters tape , 7 dollar spanner wrench and an old deck of playing cards and a second set of hands it was sort of a hey I don't feel like getting out the vice and vice block , lets see what we could do for a minimalist tool build challenge the mind a bit.
this kit is 629 and you would need a stripped lower reciver the kit can shop to your door and you buy a stripper lower from a local shop as an "other" they have been 50-70 dollars Anderson is a decent name in lowers https://palmettostatearmory.com/psa...t-m-lok-moe-ept-rifle-kit-516446780.html
you probably noticed I mentioned barrel lengths shorter than 16 inches , if your overall length breach face to muzzle is not 16 inches it can't have a stock on that gun , unless the muzzle device on a 13.5 or 14.5 is pinned and tack welded or you file the paper work for an SBR short barrel rifle and pay the 200 dollar tax stamp.
pistol braces are not stocks but not a plain round buffer tube either. for a first a rifle might be a better option.
you may well have more questions now than when you asked.