8gb is bare minimum for ZFS. 32gb would be great. ZFS will use all of it.
I would use a minimum of 3 drives because then it is easy to expand later (opposed to starting with 1 or 2).
You could start with the 16tb drives if you want. Personally I like to save power and every spinning drive takes 5-15 watts of power.
What do you mean the addresses are private? Are you double NAT-ed?
As others have mentioned, tailscale or Cloudflare tunnels can solve your problem
Do you care about power consumption?
If not, grab a used Xeon workstation off eBay or Facebook. Grab a pci HBA card and you are set. Note that TrueNas needs lots of ram.
Personally, I wouldn’t want to pay power for an empty drive, so I’d setup the 3 12x drives in a zfs pool and store/sell the 16g drives until you actually need them.
CasaOS is about as easy as it gets.
curl -fsSL https://get.casaos.io | sudo bash
It provides a gui front end for docker. You can install it on any debian based system (which mint is). Combine that with the portainer app and there isn't much you can't do.
Or proxmox and then you can run one or the other or both virtually.
This video by Wolfgang has tons of good information on finding a good low power cpu.
There is a google sheet pinned at the top where people have real world power measurements.
Basically you want to find a low TDP cpu/motherboard combo.
Why not both?
I run CasaOS inside Proxmox
Instead of 3 small drives, why not go for 2 larger drives. You are sort of fighting 2 constraints. That case isn’t meant for 3 drives.
Also I really think you will regret the usb. You could get a pci to SATA connector and put 1 or 2 drives inside the case.
If power efficiency is important, then you will want to look for CPUs with low TDP.
I reccomend this video by Wolfgang
Or this one by hardware haven
They go into depth on low power home servers.
What’s your budget?
For a very clean professional look and future proofing I would add smurf tube behind the drywall and add a media enclosure
Port forwarding will work, but it has major downsides
There are far better and safer alternatives that I would recommend over exposing a raw port to the world.