I will probably go with the ZFS file structure
What do you mean "think"? You don't get a choice on TrueNAS lol
How good is ZFS at redundancy?
The... Exact same as other RAIDs I guess?
With the HDD's how much redundancy will I need if I want to use one as a fail-safe if the other dies?
Minimum of 1? Not entirely sure what you're asking
Also, I have heard that to install TrueNAS you need 32GB ram. However I have 16GB ddr4 3200mhz, is this enough?
More than enough. I got it running on 2.5GB ¯\_(ツ)_/¯
and the other to use as a caching drive.
Caching does not work the way most people think it works on ZFS
Also, will I be able to add more drives (easily) with ZFS at a later stage?
If you buy in sets instead of 1 drive at a time, yes. If not, YMMV