A 24-28mm is a nice wide angle for a 35mm camera. Like LJSLATER says when you get to 20mm and wider they are much harder to use because of distortion. They also get much more expensive.
Remember that your wide angle lens (for a 35mm camera or full Sensor DSLR) will not look as wide on your D7000 because the sensor is cropped.
A 50mm wide angle is a very nice focal length for a 6x6 like your Bronica. It would be similar to a 28mm on a 35mm camera. A 40mm would be more like a 24mm on a 35mm camera.