You might check if a simple CPU upgrade would get you there. I previously ran some 2005 Poweredge servers that came with a Pentium D processor, and it cost me something like $8 from ebay to upgrade to a Xeon and start running KVM.

Keep an eye out for people trashing perfectly good desktop machines because Windows 10 is being retired.

If you want a server that “does it all” then you would need to get the most decked-out top of the line server available… Obviously that is unrealistic, so as others have mentioned, knowing WHAT you want to run is required to even begin to make a guess at what you will need.

Meanwhile here’s what I suggest – Grab any desktop machine you can find to get yourself started. Load up an OS, and start adding services. Maybe you want to run a personal web server, a file server, or something more extensive like Nextcloud? Get those things installed, and see how it runs. At some point you will start seeing performance issues, and this tells you when it’s time to upgrade to something with more capability. You may simply need more memory or a better CPU, in which case you can get the parts, or you may need to really step up to something with dual-CPU or internal RAID. You might also consider splitting services between multiple desktop machines, for instance having one dedicated NAS and another running Nextcloud. Your personal setup will dictate what works best for you, but the best way to learn these things is to just dive in with whatever hardware you can get ahold of (especially when it’s free), and use that as your baseline for any upgrades.