Do you want to build one yourself or are you mainly interested in off-the-shelf solutions? What’s your budget? Do you run your services as containers? Do you need hardware acceleration for streaming with Jellyfin/Plex?

I would like to already have some redundancy, can I use the hard drives as they are or will I have to do something to them besides adding other hard drives?

Why do you want redundancy? To keep your data available or to keep your data safe?