You don't need to get the lens in order to determine how fast it needs to be. You can meter the scene or fire a few test shots with any old lens, see how far from ideal your ISO and shutter speed are and then figure out how many stops you want to gain from the lens you choose.
e.g. if at f/5.6 you end up shooting at 1600 ISO and 1/30, but you want to shoot at no more than 800 ISO and no slower than 1/125 then you'll need to gain an extra 3 stops of light from the lens. That means you'll want f/2 or faster.
FWIW here's a shot from an am-dram theatre with my 50D and 85/1.8 lens at 1/125, f/2, 400 ISO, so I think I was quite fortunate with the lighting.
With an f/2.8 lens I would have needed 800 ISO, as I was already pushing my luck with the shutter speed. An f/4 lens would have required 1600 ISO and an f/5.6 lens would have needed 3200 ISO. Obviously the tool (lens) you need to get the job done does depend on several variables - light levels, distance, field of view required, ISO performance of your camera, activity levels on stage. A prime may be fast, but pretty retrictive when it comes to composition. You may also find problems with very shallow DOF if you only throw big apertures at the problem.