You can buy a nice Bushmaster at Walmart for ~800.
Or for a few $$ more, get a really nice CMMG.
They are all pretty good. If you are an average shooter, spending more won't get you much more. Start out with one of these basics, shoot umpteen thousand rounds, getproficient, and you will figure out what upgrades, if any, you want. ARs are modular. They can easily be modified by practically anybody.