Okay yeah, I've mostly fallen in love with Proxmox and Fedora server depending on the use case. I used to be a big Windows guy but that's just too much.

If you want to experiment with things like installing operating systems and virtualization, id suggest Proxmox or Fedora server. If you want plug and play and are more just wanting to get apps running, then I'd consider Umbrel, StartOS, Unraid, or Truenas, in that order. They all have app stores and mostly "just work" for deploying software on your own hardware.

For hardware, well I started off on consumer stuff, then like all computer nerds family and friends dropped off their old store-bought junkers and I repurposed them into servers as a kid. Most tiny labbers like the mini computers, think intel NUC. Now many vendors make them like HP, Dell, and Lenovo. Just tiny computers with more than enough power to get jobs done. They're low power and practically silent. I would suggest staying away from the Raspberry Pis. They have there use cases, and I do like them, but they're a bit pricey for what you get, in high demand, and when you want to modify them or they have a hardware issue (like an sd card failing) they can be harder to resolve. I just don't see the appeal for running home servers unless you really like tinkering with hardware and settings and want to be hands on. So second hand mini PCs are really the way to go imo. Some people like used laptops as well, that's fine too, however you'd want to be more choosy because your hardware and repair options can be limited or just a plain pita.

If you don't care as much about space, just about any PC made in the past 10 years is fine.

Finally if you want to get into real server stuff, I'm partial to second hand enterprise servers. A used Dell r330 or r240 rack mounted servers are going pretty cheap on Ebay and are more than enough for home labbers. While more complicated to understand, it's a great learning process and there is plenty of support on places like reddit and youtube for now. Those machines specifically are very quiet and power efficient for rack mounted gear and go for a good price. The important part about these machines is, they were meant to be run 24/7/365 without human intervention, and they mostly do that. I'm partial to Dell equipment because, 1. there's more of them, so machines and parts are cheaper and readily available, 2. they're far more user friendly, 3. they have more information available, 4. information, software, and manuals are free to the public. HP makes you pay for access to this stuff including firmware updates. (this may have changed recently)