If you could find an off-the-shelf ADC with those capabilities, I'd love to hear where. Last time I checked, I was unable to find anything that would financially make sense compared to just buying the entire oscilloscope, but I may not know where / how to look.

The EEVblog teardown of the DH800 shows that the ADC is a custom Rigol part. Whether it's a rebadged ADC or not is unclear, but I have my doubts that the price point of the Rigol can be hit with off the shelf components from the North American supply chain.