Except old machines tend to generate a lot of waste heat. Even if all you need is a text interface, you'll be better served by something modern sipping the milliwatts.
My accelerated Amiga 1200 draws some 15w.
In contrast, my Ryzen 5800x3d based machine idles at some 75w.
If it's about interacting with an instance of chatGPT that's running in the cloud, the Amiga would be far more efficient.
There are also modern low-power devices, like Raspberry Pi, that offer a better power/speed/price ratio (i.e. it's probably less wasteful to run a C64 emulator on an RPI than to run the original C64)
Seems like >30 EUR
If the original Raspberry Pi isn't available, the various "alternatives" still exist: for example OrangePi.
Your Ryzen 5800x3d is irrelevant for this context.
That's not what efficiency means.