Well the "critical/irreplaceable" qualifier is kind of important - that's why I specified it. It's up to you to decide what that means for your data and risk tolerance.

For everything that I consider irreplaceable, I'm not comfortable with anything less than 100% duplication on the live system, and also an external offline backup. Even with enterprise-grade hardware, I don't trust storage devices, they're all destined to fail. Storage is the most important part of a data server, and should be treated like it is constantly trying to light itself on fire, like a library with wooden shelves lit with torches. All the other components are just as unreliable, but more expendable.

Admittedly, when you account for hot standby drives, the cost is that my storage availability is only 40%. A 16TB storage pool is in fact 5x 8TB drives: 2 mirrored pairs and 1 standby. On the plus side, thanks to ZFS and TrueNAS those drives can be whatever, no need to worry about RAID considerations for matched drives. SAS or SATA? who cares? They can even be different speeds, ZFS will compensate. Buy whatever is the right size for the pool, let the system deal with the other issues.

Can you put a price on stability? longevity? peace of mind?

The server hardware is all old repurposed stuff - a desktop build from 2013, secondhand data center surplus, refurbished/used drives, none of it's worth much and almost all of it is well past end of life, any part of it could fail without warning at any moment. It doesn't matter, it is highly reliable because the redundancies have redundancies, including the boot drives. Trying to squeeze every bit of value out of the hardware is a losing game because it's basically worthless anyway, but the same would be true if it were all new top-of-the-line gear. The hardware is expendable, the data is not. Buy more cheap hardware, plan for it to explode, learn how to build stable, recoverable systems.