The computer should be able to run a reasonably modern version of macOS, Windows, or Linux, but doesn’t otherwise need particularly good specs.
A used mac can work quite well, but does need to be new enough to run Inkscape 1.2.
There are plenty of cheap windows machines out there, including low-end mini-computers. Here are a couple of examples of types that seem to be available now and would likely work fine (once you add a keyboard, mouse, and compatible monitor):
There are similarly priced Windows laptops as well.
If you’re comfortable using it, a low-end linux computer like a Raspberry Pi can work as well.