I would be looking at something from 3500 - 4500 ANSI Lumens for this to start with.

But most theatres will have something in the 6k - 10k range already... 

More important is likely the focal length of the lens you get, to make sure your image can cover your whole scene... 

Likely a unit of 4.5K or over is the only option to have interchangeable lenses, so I would start with something in that range.

If you're all black except the projector, then you probably dont need to worry about contrast too much, but if you are finding that you can see your black output, try switching from an LCD unit to a DLP unit.

However, how are you tracking the dancers ?? Infrared ? In that case, you will likely want a LCD unit, as it gives off less IR itself (unless you are planning to use the IR from the projector to cast light on the dancers in order to have them illuminated for the camera tracking... oh, clever!  ) 

Hope that helps...

~ Jesse

I'm sure I'll probably be needing a commercial projector in the future, but for now I can at least try this one for my first test.  I'm starting small and planing on working may way up to larger productions.  Fortunately the amphitheater where the dance will take place has a professional projector that I will be able to use, but the only chance I'll have to get familiar with it is the day of the show :/