Selfhosted

It's been a few of years since did my initial setup (8 apparently, just checked); so, my info is definitely out of date. Looking at the Ubuntu site they still list Ubuntu 16.04, but I think the info on setting it up is still valid. Though, it looks like they only list setting up a mirror or a stripe set without parity. A mirror is fine, but you trade half your storage space for complete data redundancy. That can make sense, but usually not for a self hosting situation. A stripe set without parity is only useful for losing data, never use this. The option you'll want is a raidz, which is a stripe set with parity. The command will look like:

zpool create zpool raidz /dev/sdb /dev/sdc /dev/sdd

This would create a zpool named "zpool" from the drives at /dev/sdb, /dev/sdc and /dev/sdd.

I would suggest spending some time reading up on the setup. It was actually pretty simple to do, but it's good to have a foundation to work with. I also have this link bookmarked, as it was really helpful for getting rolling snapshots setup. As with the data redundancy given by RAID, it does not replace backups; but, can be used as part of a backup strategy. They also help when you make a mistake and delete/overwrite a file.

Finally, to answer your question about hardware, my recollection and experience has been that ZFS is not terribly demanding of CPU. I ran a Intel Core i3 for most of the server's life and only upgraded when I realized that I wanted to game servers on it. Memory is more of an issue. The minimum requrement most often cited is 8GB, but I also saw a rule of thumb that you want 1GB of memory for each TB of storage. In the end, I went with 8GB of RAM, as I only had 4TB of storage (3 2TB disks in a RAIDZ1). But, also think about what other workloads you have on the system. When built, I was only running NextCloud, NGinx, Splunk, PiHole and WordPress (all in docker containers). And the initial 8GB of RAM was doing just fine. When I started running game servers, I stared to run into issues. I now have 16GB and am mostly fine. Some game servers can be a bit heavy (e.g. Minecraft, because fucking Java), but I don't normally see problems. Also, since the link I provided mentioned it, skip ECC memory. it's almost never worth the cost, and for home use that "almost never" gets much closer to "actually never".

When choosing disks, keep in mind that you will need a minimum of 2 disks and you effectively lose the storage space of one of the disks in the pool to parity storage (assuming all disks are the same size). Also, it is best for all of the disks to be the same size. You can technically use different size disks in the same pool; but, the larger disks get treated as the same size as the smaller disks. So long as the pool is healthy, read speeds are better than a single disk as the read can be spread out among the pool. But, write speeds can be slower, as the parity needs to be calculated at write time. Otherwise, you're pretty free to choose any disks which will be recognized by the OS. You mention that 1TB is filling up; so, you'll want to pick something bigger. I mentioned using spinning disks, as they can provide a lot more space for the money. Something like a 14TB WD Red drive can be had for $280 ($20/TB). With three of those in a RAIDZ1 pool, you get ~28TB of storage and can tolerate one disk failure , without losing data. With solid state disks, you can expect costs closer to $80/TB. Though, there is a tradeoff in speed. So, you need to consider what type of workloads you expect the storage pool to handle. Video editing on spinning rust is not going to be fun. Streaming video at 4k is probably OK, though 8k is going to struggle.

A couple other things think about are space in the chassis, drive connections and power. Chassis space is pretty obvious, you gotta put the disks in the box. Technically, you don't have to mount the disks, they can just be sitting at the bottom of the case, but this can cause problems with heat shortening the lifespan of the drives. It's best to have them properly mounted and fans pushing air over them. Drive connections are one of those, you either have the headers or you don't. Make sure your motherboard can support 3 more drives with the chosen interface (SATA, NVMe, etc.) before you get the drives. Nothing sucks more than having a fancy new drive only to be unable to plug it into the motherboard. Lastly, drives (and especially spinning drives) can be power hungry. Make sure your power supply can support the extra power requirements.

Good luck whatever route you pick.