For a first machine a used Mac mini, especially one that precedes the T2 chip(although that’s not a deal breaker) is probably the best bang for the buck, solid hardware that will get what most people really want from a server unless they want a full on homelab, and they are easy to find cheaply on eBay. Also comes with the advantage of being able to run OSX with fewer hoops if you had a specific use case for that(running blue bubbles in the background or syncing to iCloud… mostly just convenience stuff if you have a leg in that ecosystem could also make a potential slow migration away less irritating)

If you can find a cheap NUC first tell me where because they are great options

Lenovo think centers can be found refurbed for under $100 too and will also be available for a long time because those fuckers were in every bank, hospital/drs office, and all manner of non-tech related offices for years and years.

Or you could be like me and jump two feet in with a used enterprise server, I dunno if I’d recommend this but I do know a lot more than I did when I started and have tons power and capacity to expand. And I’ve gotten more than enough use out of them to justify the $300ish I paid for my Poweredges plus electric bills. But do your research it took me a year to find documents on how to bypass the idrac drive virtualization bullshit and my power draw significantly dropped afterwards