Is non-ECC RAM fine for a Celeron NAS?

Probably yes. ZFS checksums catch bit flips in RAM before they hit the disk, and the error rate for modern DDR4/5 RAM is very low—about one bit flip every few years per gigabyte. On a Celeron NAS with 8-16GB of RAM, you’d be unlucky to ever hit it. Save your money for a UPS or extra backup drive instead.