For performance per dollar nothing beats used enterprise gear due to how little you can pick it up for on eBay. Now if you live somewhere where electricity isn't stupid cheap or you don't have a good way to mask the sound of a 1000 angry hornets, then enterprise might not be the way to go. Dell SFF PCs can make good servers. You can also go a long ways with just humble raspberry pis, get a whole bunch of them and you can use that to learn K8s too

Something to keep in mind, those poweredges are loud. If you don’t care about the noise or if you won’t have it turned on all the time maybe it doesn’t matter. There are ways to get the server quieter, but if noise is a concern then it would be easier and maybe even cheaper to build a server yourself. You don’t need a Xeon to do VMs, you don’t need a metric fuck ton of ram, don’t feel like you need “enterprise” level of gear to learn and have fun.

I am using an Intel NUC with 32gig ram. Super efficient.

Look around the used market in your area. Likely can build a cheap baremetal system.

It might be best in the learning stages to keep your lab seperate from your daily driver machines, incase you mess up.

The top question on yer decision tree is :

is zfs part of it or not ?

If it is, you need either a midtower/nas case with a decent amount of sata ports - like 4 or more. Or install a HBA in a pci slot which implies again a midtower or enterprise server. So if you want ZFS .. all PI's and nuc/TinyMiniMicro's are out.

But you can do all the rest with them : linux, vm's, containers, reverse proxies, clustering.. you name it. Just know your backup strategy will likely involve rsync which is fine. But factor in backup in yer design from the getgo, not just an afterthought.