I’ve run retropie on a 3Ghz Pentium 4 with 4G of RAM and it all works. You don’t need too much to get it all going.
With modern equipment, I have an AMD A6-9500 with 4G RAM and built in graphics and that is more than enough
I’ve used lubuntu 16.04LTS on really old builds.
There is a lubuntu 18.04 in 32-bit if you want it. Note that 18.04 needs to be set to using Wayland at time of writing because Xorg gives libretro a hard time.