I've done a bit of dance photography, albeit under stage lights. A 50 f/1.4 (or f/1.8) is a very good idea as light is very limited. Shoot wide open and don't be afraid to push your ISO as high as it goes! I don't find the shallow depth of field a problem really, if you're shooting full length it's enough to get a whole person in focus on a crop camera.

I shot most of the following at f/1.4 ISO1600 (highest I could go at the time) and shutter speeds between 1/125 and 1/500 depending on the lighting. I was in TV mode as I wanted the shallow depth of field for effect, and I was aiming for little motion blur. Motion blur can really add to the sensation of movement in a dance shot but it's very hit and miss! No flash was used:

You say "spontaneous action, no way to stage anything or prepare" however unless you're photographing some totally wacky freestyle routines the dancers are all moving to a rhythm that enables you to anticipate their movement and time your shots. The above photo was the slowest speed I used as the lighting was worst, at 1/125 you have to time your shots (and expect a lot of missed ones!)

If you're photographing at a reasonable distance you should be able to get people's whole body in focus (though you may not get more than one dancer in focus unless they are in a line).

This shot was under a spotlight, so I stopped down for extra sharpness (and still mananged to overexpose!)