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.
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.
Throw FreeBSD on it and add a couple lines to /etc/exports and rc.conf and it's a NAS right out of the box
if you are after quietness and "power" then an old workstation is a great bet. They normally have space for at least 4 fullsized HDDs, the more modern ones have lots of PCI space for nvme-ssd cards (for space) and some have lots of lanes for speed
They also tend to come with SAS/whatever remote nvme is called/SATA expnasion options
The down side is they are not as space efficient, they also tend to have 60-120watt CPUs, so expensive to run
Along with various other devices (including a large Synology which I wouldn’t buy today), I run Proxmox on a small two bay+two nvme Terramaster. I have a bare bones Ubuntu LXC running Samba configured for Apple Time Machine, an VM running Scrypyed, and PBS for Proxmox backups. Nothing on it is critical so I don’t bother with any storage redundancy.
Recently replaced the internal USB boot drive with a small NVMe USB enclosure; using a 90-degree USB connector and using a dremel to sand away an opening for the cable to come out so I could mount the enclosure externally.