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.
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.