Most ARs now made are chambered in 5.56. Supposedly the 5.56 can use the .223 Rem round but not vice versa. Read up on that a little as it ain’t necessarily so. The accuracy of an AR depends on the ammo it “likes” more than any other factor. I have two Mossberg ARs that, according to the experts, should not shoot 75 grain bullets accurately but lo and behold, they do. At least they shoot Hornady Boat Tail Hollow Point Match bullets well. They do not shoot well with a few brands of 55 grain bullets that I’ve shot through them.
I have five ARs, two in .223/5.56, two in 6.8 SPC, and one in .450 Bushmaster. Two Mossbergs, one S&W M&P, one Bushmaster, and one Delton upper with an I-can’t-remember lower and a parts kit. That last one as well as one Mossy started life as a 5.56 but both were rebarreled to 6.8 SPC because it’s a much better hog and coyote killer in my eyes.
Buying an AR these days can really be confusing because everbody makes one. You can buy them from $500 to as much as you want to pay, at least before Covid. I have no idea what the cheapest sell for now because I’m not in the market, but the recognized brands should do ok for you. My Mossys were about $500 each several years ago, (they have no forward assist or dust cover), and I’ve never felt slighted in the least since I’ve never had to ram a cartridge home because I was crawling through sand or mud. If you take care of your guns you won’t miss them either. Good luck in your search.