Air gapped, no Internet access. I don’t use Internet services for any of my stuff though so I can get away without direct Internet access

I just happen to have a spreadsheet that covers compute density for all models of dell, HPE, and supermicro…

Your cost/density sweet spot is going to be the 2u/4 node platform from Dell or supermicro that use the xeon e5-2600v4 or scalable v2. There’s a wide selection on eBay for this stuff, so it’s definitely available. At 16 cores/CPU, 128 cores/2u unit, you’ll need 6 units.

For the dell fx2s with 4 x fx630 nodes, 256gb memory/node, 32 cores/node, I spec’d one out for $2400 before storage and GPU

Why not VMs? Dell poweredge fx2s with 4 x fc630 or any other multi-node server will not only give you density but also the ability to scale out with the option of just turning off one or more of the nodes. There’s also the Hyve Zeus 1u servers that run pretty quiet, and can also scale out depending on how many you have turned on. They’re absolutely no-frills, only has room for two 2.5" drives, but it’s supermicro-based so there’s plenty of documentation.

My optiplex 9010 sff is what I use for experimenting with services and as a staging area for moving VMs to my main lab because it’s air gapped. At max load it runs at 140w but it has a GTX 1650 that I use for gaming as well.

Otherwise the rest of my lab is only turned on when I’m using it or forget to turn it off when I leave the house. When I get a laptop again I’ll leave it on more. None of it is more than $150 to replace though. It’s a Hyve Zeus, Cisco isr 4331, and a catalyst 3750x so nothing heavy, just a little loud.