Buildapc

I'm hoping to find the best bang for the buck components whether it's used or new. I'm located in the u.s. and will look for deals on eBay. I found a case on FB marketplace for $25 already and have a cheap power supply that claims it's 700w. I'm just not sure what GPU/CPU would be, I figure it should be a ddr4 system to keep costs down. I would be open to a pre built office pc and slapping a GPU into that as well. This would be for "lan" type game nights just to have something that can play pc games as an option.

Edit: I won an eBay auction for an MSI MPG X570 Gaming Plus for $60.