Guidance for entry level enterprise equipment

My current server is running mid-tier consumer level hardware in a cheap Rosewill rackmount. I built it to replace a drobo as a NAS but it opened the door to the selfhosted/homelab hobby and now its doing a lot more than I originally spec'd it for. I've made upgrades here and there but I think my 5 year old i5 is my remaining bottleneck (go figure).

I'm considering moving into used enterprise hardware but I'm finding out its a different market than consumer components. Can anyone here provide some newbie-level guidance on what and where to buy?

I'm currently running 15 SATA drives and a pair of NVMe drives. The server provides media access for Plex with hardware transcoding on an old Nvidia 1080ti (iirc, not at home right now). The Plex server has a half-dozen users who access it. It also runs Tdarr to automate compression.

In addition to the media heavy lifting, theres general document management, backups, and some other lighter media management for ebooks and comics. I've experimented with other various other services but have turned most off to streamline what's using the processor. The video management seems to be where I'm feeling pressure the most though.

Any suggestions on where to gather information and build out an updated box? I don't think I'm googling the right questions. I seem to get a lot of outdated info from multiple years ago. If I should also just stay in consumer land but get beefier specs, that's fair advice too. Thanks in advance!