I was in Alaska taking photos of bears, etc. and I was glad to have the extra reach of the 600mm f/4 IS. I used the Really Right Stuff gimbal head but I've used the Wimberley before which is a bit smoother, lighter, and I highly recommend it. I have the 1Dx now but I had the 1D Mark IV and if budget is an issue, I think that the 1D Mark IV is a great body for your purposes -- it's also a 1.3 crop factor.
1Dx, 5D Mark III, 5Dc, 40D, G1X, 15mm f/2.8 fisheye, 16-35mm f/2.8L II, 24-70mm f/2.8L II, Sigma 35mm f/1.4 DG HSM, 50mm f/1.2L, 70-200mm f/2.8L IS II, Sigma 85mm f/1.4 EX DG, 100mm f/2.8L IS, 135mm f/2.0L, 200mm f/2.0L IS, 1.4x III, 2x III, 600EX RT (3), Einstein