500 f4 & 7D, if you can handle the weight. Otherwise, as Henrik said, the 300 f2.8. The 500 of course has a massive reach advantage when adding extenders.
If you go for the 500 f4 & 7D, but find you are too close some of the time, I would add a 5D or a 100-400 to that combo; a used 5D being cheaper.
I prefer the 7D over the 1DMk3, and I don't believe a Mk3 image cropped to equal a 7D image will give you the same quality.