Yes, just powered so the system is aware of it.

Make sure you scrub weekly. The probability of a second device failure is higher than you think, since it can be triggered by resilvering. I would also make sure you have a spare at hand. With four drives I would run a stripe over mirrors pool. And have a matching spare sitting ready on a shelf.

Hardware RAID host adapters can handle hot spares OS-agnostically, but for zfs these days it means Linux or *BSD. For a NAS I would go with a BSD distro, for hyperconverged Proxmox on top of Debian.

I can explain in more detail later.

A hot spare is a spinning disk that is known to the system and is automatically added to a RAID/pool when a disk there fails, and then triggers rebuild/resilvering of the RAID.

A cold spare is a disk added manually by the user.

Use zfs. Use RAID-suitable (TLER) drives which limit retries before reporting failure. Use stripe over mirror pools (RAID10 equivalent). Buy a spare in advance. Hot spares are immediately starting rebuilding after failure. Scrub regularly (crontab). Always remember: RAID is not a backup.

Protectli sells opnsense firewalls with SFP+ support. Make sure these can handle gigabit data rates.

You can also terminate SFP with a switch and route that at L2 (VLAN) level to your firewall’s NIC.

I would go with a refurbished Lenovo TinyPC with a quad/hexacore and some 16 GB RAM. These used to go for 200-300 EUR, but this might have changed recently. I run an AMD Fujitsu thin client as my opnsense firewall.

How expensive is your kWh? What about noise? What about cooling in summer? Your SAS drives will fail, so make sure you have spares.

I personally would look into suitable used Lenovo TinyPCs and a passively cooled switch to cluster them. Consider your nodes expendable. Cluster storage at node scale rather than RAID.

Run a libgen mirror (over VPN). Pin docs on IPFS.

My first interest was in running Unix for uucp Usenet, early 1980s. Never happened since I was poor, so it took DSL availability some 20+ years ago to run a Debian server at home. Around 1997 I ran my own Linux box on a university network, which ran a web server.

I run Ubiquity AP (used to be flashed to OpenWRT but now stock) as well as Mikrotik, all local. Firewall is opnsense. NAS is FreeNAS, but not really use it at the moment. zfs is great. If you really need 48 port L3 switches, look into whitebox. I use used enterprise gear for lab, too much noise and power draw and no real use for terabit L3 at home.

Use opnsense instead.

You can install Proxmox on top of Debian. I always do that.