I'm sure there's better options now but the HP ProLiant MicroServers (used).

They support ECC ram, 4 caddies, one extra PCIe slot, and to my knowledge you're not cpu limited for a zfs file server usecase.

Keep in mind though, all you need is linux* support, iDRAC, ECC if you're a snob, and drive bays ... and that's basically any free server.

In my extremely opinionated opinion I would only get used enterprise server gear, because a zfs file server will just work unless hardware fails. And a UPS.

*ZFS will be a more natural choice on FreeBSD. It's better documented, and will meet Linux 1:1 in hardware compatibility for this.

I'm still running an old Gen 8 MicroServer. Modern drives can actually saturate the SATA controller, and because it only has a single PCIe slot I can't add both a 10Gb NIC and a storage controller - I went with the 10Gb NIC.

It works well enough though and has lasted me over a decade at this point. 16GB DDR3 ECC, an old 4 core/8 thread Xeon, 4x14TB drives and the Mellanox NIC.