Researchers have built server prototypes that re-use old components

Would love to know where, I want to convince friends they should get a home server, even if its just to use it as a NAS

Apart from eBay and specialized reseller websites you can actually find some good deals on used enterprise gear onAmazon through third party stores. I've bought a few different things and had excellent results universally. The real trick is just knowing what to search for, which servers or computers are popular in enterprise and on the end of their lifecycle. Dell, HP and Lenovo servers and mini PCs that are sold to enterprise and are a couple generations old are what to look for.

99% of people will be better served by a single consumer PC instead of enterprise gear, don't convince them to buy a poweredge!

OOBM is a must have.

Yep. I am very satisfied with my do-almost-everything Lenovo M80S gen3. It came with an i7-12700 (12 core 20 thread) and I've dropped in 96GB memory, a 10gb network card, and an LSI HBA connected to a used SAS disk shelf.

It runs a dozen VMs and sits almost idle most of the time unless I'm experimenting with CPU LLMs. My one quibble is that it's small form factor and has limited PCIe lanes so installing a GPU is complicated.

The whole setup including the M80S, the disk shelf, an old Brocade network switch, a Unifi NVR, and 12 spinning rust disks uses about 200W total, which is about $30/month in electricity.

In a data center, sure. At home it's much less interesting and you can fake it with a NanoKVM or PiKVM for fairly cheap. Also a lot of "business" desktops with Intel processors will come with vPro, which is almost but not quite the same as a true IPMI.

if you absolutely must, you can still buy standard form factor mobos from supermicro or asrock with a BMC and put them in a standard, quiet desktop case

I got 3x Dell R720 2U servers off ebay a year or so ago, they were $300-ish landed. Dual 6 core CPUs, 256GB RAM, no drives. I got 2 of them for work for our dev/stg cluster, and the price was so good I decided to get one to set up a homelab.

Especially if they want to run it for a desktop use. Decades ago I was running a small data center operation and would sell our old servers into the local Linux community. Had a guy buy one and then bring it back, dissatisfied with the graphics performance. "Like, whatddya mean, it runs the serial console at 19.2k..."

An old laptop can make a great homelab server if you don't need that much processing power. It's quiet, and it's got a built-in KVM and battery backup.