The following is what I would call minimum for general 3D emulation and light gaming. You could get a graphics card used for cheaper but I was just going with what I could see on amazon. These Dell Optiplex PC's can be found on Craigslist/Offer Up/Facebook for cheaper as well.
Graphics
Radeon RX6400 - $150
or
Nvidia 1630 - $150
PC:
Optiplex 7020 - $100
I got one of these on Amazon recently: Optiplex 9020
Came with a 500gb drive, but I put a 1 tb SSD in it. Mine came with 8 gigs of RAM, and I'll likely put another 8 gigs in there too.
I also got this video card: XFX Speedster SWFT105 Radeon RX 6400
In windows it'll run everthing up to stuff like Street Fighter V, Killer Instinct (2013) and Tekken 7 without missing a beat. And in Batocera it'll run Dreamcast, PS2, N64, etc etc etc.
$300 and you've got yourself exactly what you need. Granted its not a microtower, but to put a video card in the pc (at least a low profile one) will need that midsize form factor.