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.