Maybe something like the new Sigma 17-70 f/2.8-4 ($500) paired with a used 430EXII ($200) would work for you.
I take pictures of my dog more than anything else. While a fast lens is great for taking shots of him inside in low light, even a 1.4 is not fast enough sometimes. You also have to think....as you stop down to 2.8 or lower, your DOF becomes more shallow. With dogs, the distance between the nose and the eyes is even more than it is with people. So, what happens is this....using autofocus, you catch the nose a lot, and shooting at 2.8, you basically miss focus a lot. You get a shot with a nose in focus, and the eyes way out of focus. If you add a flash to the mix, you can move your aperture up and really get the shot you want.
The zoom is nice for dogs since they move so much, but a prime gives you a special look. If you are going to go with a prime, I would go with something 35mm or less. Going with a 40, 50, 85, or longer will prove to be frustrating when trying to frame your shot....from my experience, working with dogs is usually done in close proximity.
So....maybe get the best lens you can in your price range, and save for a flash.
6D, 5D, 20-35 2.8L, 135L, 70-200 2.8IS L, Sigma 50 1.4, Sigma 105 2.8 macro, SMC Takumar 28 3.5, SMC Takumar 50 1.4, Super-Takumar 105 2.8