Recently I managed to find a decent dual-processor Dell Precision T5600 on ebay for ~90 euro. With 2x Xeon E5-2630 2.3GHz | 6 core each, 8 DIMM slots for ECC RAM, and Intel 82759 network.
It came with just 8 GB RAM, but second hand options for 1333 or 1600 DDR3 RDIMM ECC memory were plenty on the local market, so I managed to find additional 56 GB for 60 euro (for the 8 DIMM slots the options are 1 GB, 2 GB, 4 GB, 8 GB, and 16 GB ).
The latest BIOS rev is A19, but in order to upgrade the BIOS you need to disable the Intel virtualization options in the BIOS first …. it took me two days to figure it out, because the processes finishes without any error or warning, but you end up with the old version. If you do not upgrade the BIOS to the latest release, you will not get the disk controller enabled to configure any RAID arrays.
I installed FreeBSD 13.1 with zfs and made software 1+1 RAID with 2 disk. Everything is supported and works like a charm. The dual-processor setup is a little bit power and cooling hungry, but it is bearable.
The official manual:
https://dl.dell.com/manuals/all-products/esuprt_desktop/esuprt_dell_precision_workstation/precision-t5600_owner’s%20manual_en-us.pdf