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Yes, adwords CR would give you that answer. The budget required depends on so many factors. But you can reduce the list of KW sampling the complete list. But at least at macro level if you discuss that with someone from your client who knows his market and his consumers you should start getting an idea. Logic+common sense is a good start. I would analyze that before to start changing the website. But if you do the opposite is not that you are going to break any porcelain. Duplicate content is not like a manual penalization, as far as I know, once you fix it and google crawl the new version the ranking is updated.

In the interests of closing off the question, I've decided to keep the existing content on the same URLs and refresh the UI so it provides a better platform for hosting the high quality content. My rationale is that I did find backlinks pointing to some of the content which would be a shame to lose, and setting up so many 301s to a new location did not seem like a good use of time.

So glad to help, George!