I'm a Canon shooter. As far as lenses - I love my 24-105 for all of my studio work and some outdoor stuff. Outdoors I usually use the 50 1.4, just because the bokeh is so yummy. I rarely shoot in low light, even when I have a session in someone's home (which doesn't happen too often), but if I using natural light I use the 50 1.4. Love how sharp and contrasty it is!

As far as ISO - I shoot ISO 100 in the studio and 400 or 800 with natural light.