Most anything is powerful enough to run channels. It depends on your use case. If you need multiple remote transcode streams, then you need more cpu/gpu power. And/or you also want the fastest possible processing for com skip.
Running Channels server off a SBC like Pi4B, or on other lower powered dedicated NAS devices, are more than enough for most users, especially if you are the only user.
Sure, using an old "office pc" like a Optiplex or whatever will be fine, and those are dirt cheap or often free to find almost anywhere.
However, its the raw power usage and efficiency over time that is bad with those units.
Often idle around 50 watts and loaded over 100.
A pi4b only uses around 6 watts under full load and around 2 watts at idle.
A mini/micro pc, like a Intel NUC or off brand clones, are often low power chips, ranging from 10watts to 35watts+ under load.
If cost of electricity is low in your area, or you just don't care about your power bill, then a powerful machine may be practical enough for you. Just as long as you also have proper cooling ability. Those larger more powerful machine put off lots more heat, so you don't want to just hide them in a cubbard to fry. Also, fan noise could be an issue.