Nice start and solid choices so far!

Definitely recommend looking into Docker, it’ll make cross-platform conflicts and migrations near effortless. Repurposing unused hardware is great, but can also be inefficient or bulky, so a hardware upgrade might be in your future eventually. ;)

If you are interested in automating any media retrieval and/or organization, which I gather you might given you have Plex, look into the *arr ecosystem: Sonarr for TV, Radarr for movies, plus others for books and music and pretty much anything else you can imagine!