I just bought a new computer, and I have been brooding on what to do with the old one.
I have decided I still want to use it. It is still a fairy powerful computer.

I want to use it as a storage and media slave. I'm thinking I'll connect it to the old (720p) projector in my bedroom. It already have a plex server installed that I occasionally use with my PS4 connected to the TV in the living room.

Besides plex, what else should I need to turn my old desktop computer into a network storage device, as well as a media player and media server?

I don't wish to install a new OS if it can be avoided, and keep the existing Linux Mint.
How would I go about installing a fitting desktop environment and software to fit this new role?

If installing a new OS is the only option, what Linux distro should I go for?