If you have old parts, use those, it’ll probably overkill. Most server stuff isn’t very resource intensive, so a little goes a long way.
If you’re buying something new, I’d recommend something small, like a Mini PC or an N100 rig. 16GB RAM is probably enough, and anything with more than 4 cores is probably overkill. A dedicated GPU is unnecessary, something with a modern-ish iGPU will be plenty to transcode video.
Yup, my first NAS was my first desktop PC, and I’ve upgraded it as I upgraded my desktop. My current NAS is still running my original Linux install, and currently has a Ryzen 1700 and Nvidia 750 Ti… Y desktop has a Ryzen 5600 and an AMD 6650XT, and I’ll upgrade my NAS to that when I upgrade my desktop.