That’ll highly depend on what clients you use and if they can stream your media as-is or need the server to transcode it. If no or very little transcoding is needed, you can go with pretty much every NAS or a Raspberry Pi as a server. The moment transcoding is needed, you’ll need a more and more powerful computer for the job… an adequate GPU or iGPU can soften that but hw-accelerated transcoding won’t work for all use cases (e.g. if you have subtitles that depend on being burned into the video… that job is solely with the CPU).