I have no issues with older bodies. I've tried to joke before that the previous bodies were "awesome" and the newer bodies were "awesome(er)".....To me this gets down to specific reasons and purposes and I think you outlined your intended usage very well and the older bodies fit. When I bought my 5DIII I did briefly look at picking up a used 1DsIII instead. At the time, used prices were a bit higher, but it was still pretty comparable. I don't recall all the reasons why I went with the 5DIII, but I do recall that they were on the specific reasons why I was upgrading from the 7D. For example, I did want the higher ISO performance as low light photography was one of the reasons I was making the upgrade. I am also in a different situation whereas you have multiple bodies and can choose which body is best for your usage. I want to have a single body that is the best fit for all my intended uses. So, I went 5DIII. But I can certainly see using an older generation 1D series body.