What you did not address in your requirements is the network speed. If it is plain old gigabit than almost any cheap motherboard cpu combo will do. Additionally do you want ecc or satisfied with zfs built in safety features and redundancy? Thirdly energy consumption... How much is too much?

Old enterprise gear can be silenced pretty easy if you are willing to go the extra mile, and most have ipmi built in, and if you are lucky can get it for real cheap. Examples: 1 2 3

But the downside is the horrendous energy consumption relative to what it provides. Unless you throw away the inside and put in new board cpu etc.. but that's a whole other conversation

If complete new than you depending on the ecc choice I can advise two direction.

One. Get something like N100DC-ITX or N100M put in an additional sas/sata controller and Bob's your uncle.

Two. Get an older Ryzen pro desktop second hand and do the same as above but this time ecc is an option. Example

For both of these option you could use your old pi as a kvm

As for the case if new and hotswap or easy/fast swap is not a requirement, than a cheap Rosewill 4u will do. Otherwise get a used Supemicro server just for the case and backplane.

I would rather consider having two separate unit with a 10gbit interlink, than to have a single one. The cheapest Mikrotik only combo with a CRS309-1G-8S+IN and CSS610-8G-2S+IN would cost combined 388 USD new and fanless and around 17W combined idle power draw.