I think you should buy what you can afford.
Just like precision rifles you get what you pay for here. A cheap AR will function fine but a higher end AR is nicer to shoot. The better companies usually put more time into tuning the gas system to the buffer system making it smoother shooting, they usually have better coatings, come with better furniture, and usually a much better barrel and trigger and also better QC before letting that rifle leave the factory.
I would stay with a direct impingement rifle over a piston rifle simply because of parts availability (even through I have 2 piston operated AR's that I love).
A DI rifle can use parts from any other AR and from almost any other brand. A piston rifle usually has a proprietary gas system and bolt carrier and you can only use parts from the same company making repairs more of a pain if something ever breaks or wears out.
Just like optics, spend as much as you can comfortably afford, getting a cheap one will leave you wanting and then you will have 2 AR's, then 3, then you'll buy that expensive one you always wanted anyway.