Hardware requirements are mostly: amd64 system, 128 MB RAM or more (depends on if you want to use the GUI or not) and two NICs.
Most routers are ARM or MIPS and there is no official Whonix image for those architectures but you could try to either cross compile or compile directly on the router. Both options are going to be slow, the second one will likely be slower. I’m not sure if Whonix will currently build successfully on something else than amd64 (or i386). Also most routers are not designed for high speed crypto stuff like Tor so the speed might also be slower than usual.
Your best bet regarding easy setup, compatibility and speed is probably your unused computer with a second nic or some amd64 router board (those will have multiple network interfaces and also a lower power consumption).
There was also a thread on Whonix on the raspberry pi: Whonix for arm64 / Raspberry Pi (RPi) - #7 by deekay0