I will absolutely second that sound advice though I haven’t seen any $500 NIB AR15s for a while. buy plenty of 30 rd magazines as well.
Cabelas has been running ads every few months. 5-600 dollars. They had the DMPS Oracle on sale for 560....Slap a quad rail on it and optics and/or laser.
I also agree on the Colt LE6920. Nice rifle.
However, I spent the weekend with a S&W MP10. Tack driver at 200 yards with optics. Oh boy, what fun that was, however I did swap out the trigger for a 2 stage...