My entire proxmox stack runs off of a hard drive that I wasn’t the first user of, and at some point I thought it had stopped working so I wrote “dead” on it. When building my server I needed a drive and I tried that one and it has served me well since (it is going to fail in less than 50 microseconds)

Since you’ll want to stream content rapidly off of the drive using Plex, I recommend getting an SSD for mass storage. Since you don’t care about boot speed (since you won’t be doing much of that) I’d recommend saving and getting a cheap HDD for the boot drive. Ideally 2 of them for redundancy.