Older and or used hardware is gonna be a place to start for CPU and GPU. Used dell optiplex can get you most of the way there, then buy a decent GPU when you can. Just make sure it fits in the case and the PSU that comes with the optiplex can handle the power draw. I’d recommend a new PSU though. Dont buy used for PSU or storage is the best advice I can give.

Optiplex are not gonna get you top of the line performance or anything but it’ll be a lot better than nothing and you can always use it for something else later like a nas, a server, home theater PC, etc.