The link you posted is not working for me but doesn't matter. Thank you very much for the writeup!
I found a few of https://commons.wikimedia.org/wiki/File:HP_8200_Elite_SFF.jpg#/media/File:HP_8200_Elite_SFF.jpg on ebay, refurbished business machines mostly. I think you can fit a drive or two inside there? The mainboards have SATA connectors. Maybe I could also take the mainboard and power supply out and put them in my old tower.
Another thing I thought about was buying a https://commons.wikimedia.org/wiki/File:Intel_NUC_Haswell_(case_rear_panel).jpg#/media/File:Intel_NUC_Haswell_(case_rear_panel).jpg, put it in my old tower, add a M.2 6xSATA card and put the drives in the bays of the tower.
But probably I should just use my RPi400, hook up one or two big external HDDs (add RAID?) and start there. Then I can better decide what I want and need after testing Openmediavault for example.
While talking about the RPi and booting from USB drives. Should I buy a cheap-ish USB SSD and boot from that and not from the SD card? What's the problem with SD cards?