How do I pick an ADC to monitor the output of a switching regulator?
If you really want to do test coverage I'd do a FMEA on the CUT (Circuit under test).  But baring your ability to understand what the output will look like, why not take a board ( and replace components) and look at the waveforms when components fail or are out of tolerance.  That will tell you a lot about what you need to look for even if you don't do a formal analysis.

You shouldn't have to sample at a high frequency unless you think that one of the failure modes will actually create higher frequencies.  But even so, this is a DC power supply for knowing that the output is stable, responds to load changes and doesn't generate too much extra ripples should be easily obtained at a lower sampling frequency.

Keep in mind that if you do get a higher frequency there you will get sampling/aliazing and the ADC will stil see it as an increased "ripple".  Your sampling frequency should be set to be able to see some of the dynamical behaviour of the circuit.  So again fiddling around on a lab bench for the afternoon and destroying a few boards will tell you that as well if the datasheet doesn't.  Some datasheets do tell you want the dominant pole of the response is.