Buy a complete rifle for $400 or less ...... assemble your own lower @ your kitchen table while watching a youtube video and buy a completed upper for (all in) $400 + ..... hand pick each piece ... well that's just maddening and $$$$$$$$$
and you get what you bought assuming you don;t know what you think you want. I tend to buy mil spec several lowers around $50ish, find a deal on a lower parts kit ( I currently like Strike Industries enhanced lpk $40 - $70) JP trigger springs $10ish, Hogue grip and stock or whatever you like. Maybe a drop in trigger set (?) $100+ ............ get the parts you touch that you like and buy an assembled upper of your choice there are 100s if not 1000s to choose from. Good Luck BKG
I am a huge fan of Geissele drop ins. Never tried the JP but seen good reviews.