Never having made a film in the Ken Burns style myself, here is how i would approach the next steps on (what i know about) this film:
view post production as the place where the film will likely find its voice. as such, you'll want to assemble the foundation and structure of the film AS you acquire the Broll and illustrating material. Even if you know right where to go for all the images that you hope to use in the film, it will undboubtedly take much longer than you anticipate to get the rights to use all those stills and Broll/archival material. Of course, you may claim fair use on all that material, and decide not to pay for rights – but that is a bigger question for you and an entertainment lawyer to answer together after much research.
do this simultaneously with shooting interviews and other scenes that you want to include. But I'd recomend starting right now. You say that you've got a structure that you want to use in the film – sounds like using this big interview as the skeleton. Open up a new FCP project, and save it as "Evil01" import all the usable clips from your big interview, and start laying it down on the timeline in the structure that you imagine. How does it flow so far? Did you get everything you needed from your subject? what else do you need to add? which concepts need to (or can) be explained by other interviewees/sources? which examples in history have stills/film that you can cull from? are you finding the right balance to make the material engaging for your audience? start work on getting all those assets, and by repeating the questions above over and over for many many months, you can start to see your film take shape in the way that you want to get your message through. a journey of a thousand miles begins with a single step – and making a great documentary is in many ways a longer journey than that. you've already taken a few big steps, which is more than 98% of wannabe doc makers take, but now its time to jump in the difficult questions of how to best tell your story. it seems to me that jumping into post production, along with creating the beginnings of a paper edit and maybe a white board to outline structure – and you are well on your way! its going to be a great learning experience – and hopefully you'll have a great film in the end as well!