@Bigun is right.
Business class HPs is all I own anymore... I have one dell, and the carcass of one white box - my last white box, my main server. It stayed around longer than the rest because I required an aftermarket board simply for all the drives that were hanging off it... But now, with the advent of ginormous drives. CICERO (my server) is now housed in a more modest 3/4 tower, and I am down to 5 drives (and a system drive) right off the rack from HP (sans drives), used, off-lease, and 3 years old... Bought (traded) for 150 bucks (in value).
No one I know is building white boxes anymore. No profit in it.
BUT, I will give you some tips: ASUS, MSI, or Gigabyte. Nothing else. NZXT makes some pretty good cases with monster power supplies that can be included... Or at least, they used to.
If you are willing to wiggle on specs, loaded boards are usually the way to go, coming with board, processor, fan, and RAM usually at around a 30% savings over off-the-rack retail, but without the satisfaction of getting precisely what you wanted.
I think you are probably best off in Intel processor and board architecture and nvidia graphics. A gamer box will put a lot of emphasis on a discrete video card. butt-tons of ram, and probably an M.2 NVMe system drive... That's for a top shelf box. Dunno if that is what is needed.