Thoughts about buying a Bedini 100

Since this is all new to me:

What would be the most important replacements to do 1st?

Can partial replacements be done? - can a tech evaluate what's still good and what should be replaced, instead of a blanket replacement?

And I'm sure this seems like a foolish question - but if I listen to it before (which I'm sure I should) - how do I know how 'well' it is working, or even how 'good' it might sound with replacement electronics?

The Bedini is a fairly simple amp- so if it works at all, then the main filter caps and the filter caps on the circuit boards should be replaced. That should take care of it if the amp is otherwise in good shape. This isn't that crazy hard to do. Once sorted out, they tend to be on the smoother side as most solid state amps go and that single quality puts them near the top of the heap to this day.

atmosphere

The electronics rework sounds simple for a competant tech - but should it cost around $500?

No, the new caps don't cost much. And an hour or less to install them! Anyway, unless there is a hum problem the caps are probably still good. You can use a variac to slowly ramp up the AC voltage to avoid stressing the caps.

Power supply caps are simple, get that done no matter what. On all 3 of my old class a units, cap replacement was a huge improvement.  roberjerman is correct,  The power supply filter caps are a simple switchout on this unit.