Usually they’re normal x86 PCs with nothing unusual about them so just your Linux/BSD distro of choice. You can look up the processor model to see what crypto acceleration it can do, or see if there’s any wireguard benchmarks available.
Some have interesting processors like PowerPC, or other strange hardware, but avoid them unless interesting is what you’re after.





Old phone which is supported by (even an old version of) LineageOS, kept permanently in airplane mode.
Can be extra cheap in countries which have switched off 2G+3G networks, and old phones are more likely to have a headphone jack. You might even have one kicking around in a drawer.
Audio output quality and headphone driving capability varies – research on audio forums for what might be good. LG v-series were very good.
If you’re feeling adventurous you could crack it open and remove/short antennas & microphones and cameras.
If it’s usb-only or the audio section is poor quality, apple sell a cheap usb-c dac that’s pretty good. (get it from a reputable source – most are counterfeit).