If all you want to do is play some mp3s through a single bluetooth speaker, Plex is massive overkill for what you are describing.
As a rule, I’m a huge advocate of overkill – but, if thats all you want to do, just save the mp3s directly on to the phone, and plug it into the speaker.
That approach only wouldn’t work if a) you have more content than would fit onto a phone or SD card, and/or b) you want several people to be able to access the music from one central location.
Plex is free and will run on a potato, so just download it onto your existing computer, and the client app (or Plexamp if you are mostly interested in music) and give it a go. It’ll take you 30 mins to get everything working if you’re only using it for music.
As long as the “client” (whatever you are playing the media on) can play the media in the format it’s saved as on the server (mp3 in this case), then your Plex server doesn’t need to be powerful at all. It’s only when you want your server to start “transcoding” the media files (eg: if your server needs them in a different format or compressed to send over 5G). Even then, if it’s only mp3s, your old MacBook shouldn’t have a problem.