The question about the applicable MDA has been answered.
In the case of a higher MDA (ex: 1,000'), just remember that the circling approach is a visual maneuver. I normally start descent on BASE leg with the runway in sight to judge my vertical profile using the same timing of 20 secs corrected for tailwind. Descent rate in excess of 1,000 FPM might be required. Perfect example of this is in BUSAN (or Pusan, S.Korea). Check it out.
Just don't forget to brief your colleague accordingly. Cheers!