I just built my home lab not too long ago. The suggestion you have in your idea seems fine with the smaller Lenovo or whatever PC’s as nodes. Its best to have all matching machines if you want to try HA (High Availability) Nodes like you were eluding to I think.
Then a centralized Media Storage NAS would also be useful here. You can get one that can integrate services like Plex etc.
Then there’s another option of using one machine and Nested Virtualization. So running a hypervisor with Nodes as VM’s all interconnected on one machine.
I built my all in one machine, then decided to dedicate different devices for different purposes (IE DNS, DHCP, Firewall, VM server/File Server and so on) for diagnostic purposes if something failed I didn’t loose Everything because this is still my home network. And if wifey loses her services its bad juju.
Like @Dutch_Master recommended Pi’s would have been great for this application. I dunno if Orange Pi or other SBC (Single Board Computers) would be another viable option as nodes for a cheap setup.
Have my machine here which is WAY more than is needed for a single machine for VM’s etc. Least for my needs messing around, sadly prices have increased significantly. The ASRock Board and ECC ram were the big purchases. It’s nice to IPMI for remote monitoring and control of the server completely remotely.
As someone who spent the money on 10G nic’s etc…its nice, but not at all necessary for home tinkering. 2.5G is cheaper but even 1G is plenty for most cases.