I would set up a media server in the old case that fits your drives but set up the game servers on the lenovo.

Plex is fine for people that already have it set up but since you’re starting from scratch why not start with the free one?

Have you already bought a HexOS license? You can do more with Proxmox or TrueNAS for free.

Setting up an SMB or NFS share is the correct answer because op has stated he plans to use VMs (plural). ZFS is not sharable without it. https://pve.proxmox.com/wiki/Storage

There are forum posts going back years about it :https://forum.proxmox.com/threads/share-zfs-storage-between-nodes.144790/ https://forum.proxmox.com/threads/create-a-large-pool-of-shared-zfs-storage.94435/

TrueNAS is not absolutely required. It can be done with a different Linux distro. I use/prefer Debian.