It was designed to be extremely simple and reduced in scope to the minimum of what a processor needed. It went into space. Radiation hardened versions were made.
The original version had its functionality broken up into multiple chips. That could allow for easier repairs.
I don't know how many transistors were in it, but I doubt it's more than the Z80 or 6502.
The RCA 1802 is another one I'd consider. In fact, it will likely outlive the human race entirely, as it's in the Voyager spacecrafts.
But you won't find them in calculators just lying around that you can scavenge. Remember, the narrative driving this is post-economic/supply chain apocalypse.