For Lens choice: Visit the location beforehand. In my opinion: Use the longest focal length lens with which you can fit the whole stage with proper framing.
You want to avoid placing people in the corners, using wide angle lenses. This kind of distortion will make people look fat - no one's going to thank you for that.
Bring a tripod, so you don't have to choose the shutter speed based on how long you can hold the camera still but based on how long do people typically hold their body motionless. In my experience 1/30 will get you a safe sharp shot in most cases. You can try longer shutter speeds, if you're feeling lucky. But in any case: Take multiple shots at each shutter speed you try.
Do not rely on Auto exposure. Use manual mode and check the histogram.
It goes without saying, but I state it explicitly, anyway: Shoot RAW.
Lighting will be the hardest. What equipment do you have, and what equipment is on stage. Is there someone to operate the stage lights?
Ideally you have diffuse light from an angle.
If you have several flashes, you can place the flashes facing towards walls and use the reflection of the flashes as diffuse light source.
For 200 people on a stage, however, there would need to be several objects on the stage from which you can bounce of light. And you'd need at least 4 or 5 flahes.
If stage lights are present an can be operated: Strong light from one side, a fraction of the light (75% or less) from the other side.