Is the quest of replicating a Datron 470x calibrator totally foolish or crazy ?

[dietert1]

I think one important aspect is bandwidth. While many commercial ADC and DAC products are meant to perform at least at audio bandwidth or more, for a precision calibrator one doesn't need that. For utmost accuracy one can use time division schemes that are more like those used in high precision meters, thus reducing the switching by large factors, e.g. 1000. Also opamps work much better at low bandwidth, like 100 Hz or so.
I would not recommend reengineering nor replicating an ancient design but rather implement something similar using modern parts, e.g. those zero-drift amplifiers we now have.
Recently i studied a little a Prema 6048 8.5 digit meter. At the time they made proprietary microcircuits to implement their time division ADC scheme at 3.6 MHz: a multibit delta-sigma converter, where a 100 Hz PWM serves as multibit DAC. Meanwhile i got something similar working with a STM32L432 MCU, yet at 64 MHz. Don't need that proprietary part anymore and get almost 20x more resolution. And the STM32G4 MCU family provides high resolution PWM with an equivalent clock of 5.44 GHz or 184 pico seconds. Other brands offer similar capabilities.

Regards, Dieter