Can anyone recommend a good server for a homelab to use for a storage purpose like this?
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.