The problem with these older units is that we have come so far in terms of performance per watt that the cost of electricity quickly makes them irrelevant.
I used to run a cluster of Dell 710s that I was able to replace with a few M720q very cheaply off eBay and run the same workloads at less than 1/3 the power usage (even better at idle, since that really where these old units don’t do well).
I have seen a few converted into DAS or drawers for a rack, but you’d have to already have a rack for that to be useful or worthwhile.
As far as “just works” get something that is directly integrated with home assistant and IMO if home assistant has direct access to the radio or method of communication, the better. As others have said both Zigbee and ZWave devices can interact directly with HA using a dongle and I do so with a VM on Proxmox and it works great, I do however think the ZWave JS and ZWave JS UI integrations are far and ahead of Zigbee ZHA integration (I won’t speak on Zigbee2MQTT as I don’t use it)
Wi-Fi devices are nifty but their reliance on the strength of your WiFI, internet connectivity, DHCP working, IPv6 support make them more variable than the ZWave and Zigbee standards that really just require you build a good mesh by having devices every so often that each other can “relay” through.