[KK7NIL]

There is no Nyquist frequency in analog. The Nyquist frequency only appears when we're going between a continuous (analog) system and a discrete (digital) system.

For a real receiver circuit you'd want to sample at least once a symbol obviously (so RX sample rate equals the baud rate). In practice you'd want more than that so you can better do DSP afterwards to remove channel losses and such. For a test instrument, it's going to depend on what exactly you want to see. Most standards define a required filter shape and BW for testing and you need enough sample rate in your scope to be able to implement this (usually in DSP).