I have been invited to take some photos from an ensemble in a conservatory. I do photography a lot, but mostly outdoors and this is going to be my first experience. I have a Canon 50D with 2 lenses: 1-50mm and 17-55mm IS(which is horrible ;) ). However, I am able to rent some lenses for that night. What do u think I should get? a telephoto or a wide or both and which model is preferable.Either way because this is going to be a classical concert then flashes are not welcome indeed, so the lens should be able to handle the lights there(i think the lights are sufficient). However, if there is any special flashes for this matter, which is not going to be really annoying, please let me know. Please give me a few options of each. I apologize for my non-pro literature. Thanks

I have been invited to take some photos from an ensemble in a conservatory. I do photography a lot, but mostly outdoors and this is going to be my first experience.

I have a Canon 50D with 2 lenses: 50mm and 17-55mm IS  (which is horrible). However, I am able to rent some lenses for that night. What do you think I should get: a telephoto, a wide, or both, and which model is preferable?

Because this is going to be a classical concert, flashes are not welcome, so the lens must be able to handle the lights there  (I think the lights are sufficient). 

However, if there is any special flashes for this scenario, which would not be really annoying, please let me know.

Please give me a few options of each.