Compact business desktops like others have mentioned are great. Depending on your needs, I also like using older or used laptops. They’re still power efficient if you get a recent processor model, people sell them for fairly cheap used, and sometimes having an attached keyboard and display is more convenient than having to hook up a crash cart

Makes sense. I think you’d be fine with pretty much any modern(post DDR4) motherboard/CPU combo these days. I feel Linux hardware support is only really shakey if you’re using a SoC without upstream patches or if you’re using brand new hardware/laptops. With that being said if you’re running a lot of containers on one host have you looked into docker compose or kubernetes(k8s)? Maybe k8s is overkill for home use, but both offer support to restart containers if a health check fails. With k8s you also can spread out containers across multiple physical node, so you could just add a second RPI and “double” your resources.

For $350-500 you could easily get a used desktop and processor with 16-32 gb ddr4. But it sort of depends on your home lab goals and workloads. Do you need a lot of storage? Are you CPU bound or memory bound? Some people will suggest used Dell/HP servers, and they’ll look affordable, but keep in mind enterprise gear will eat power and is usually loud. Personally I’d go for a used AMD 5800 or 5900 processor and mobo, install your favorite Linux, and call it a day. AMD processors don’t have quick sync which makes them slightly worse for plex hosting but better for everything else.