Hey there !
Dpms , Windham , Bushmaster are entry level guns and good enough for most people. 16" or 18" barrels will do the job 500+ yards. Piston are overkill and an adjustable gas block would be the only upgrade to a gas system gun needed.
168 gr ammo works best in our guns. Optics are a must, spent a few hundred but a thousand dollar scope is nice but not necessary. Lube the gun big-time on break-in (dripping) should be good after 100 rounds.
Good Luck 👍