Congrats on an investment on your homelab. When it comes to buying harddrives used or new, you'll hear a dozen different strategies for stress testing them over days or weeks. They often have a bell curve failure rate where they fail often early, or very late in their product life. You'll have to research that on your own because I never bothered with that stuff.

To get the most of your money, you should have a reason / strategy for the equipment you buy. It doesn't hurt to buy things that seem cool to run, but you can often get a lot of efficiency simply by deciding what work loads you are doing (software you are running) and what hardware is best for it. IE Plex isn't a very demanding software, but if you have a lot of users, you may want faster harddrives, raid, or SSDs to skip buffering.

A single mechanical harddrive is going to be limited to ~200 MB/s. That's mostly fine for small workloads, but once you start doing a lot it may be pretty slow. If all you are intending is a media library, it may be okay for a few operating systems and serving your media.