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First thing I would look at when considering the changes your are is how well is the current website doing in the SERPs - does it get much search engine traffic? Is the domain name itself important to the business? I would look at keeping the domain and look at either addressing the current duplicate content issue with canonicalization or changing your CMS and pointing the old urls to new corresponding ones using 301 redirects. I have done this successfully in the past when changing the URL structure of a site and I didn't have a negative impact on search traffic, in fact quite the opposite. That way you are passing on most of the value gained by any links to the old URLS and then you can concentrate on the new CMS and all of its benefits including implementing a new user friendly structure, without having to worry about the old structure and following it..

As well as Google Analytics and Google Adsense IDs.

If you consider the meta-description field "important as a ranking factor," you're simply incorrect, I'm afraid. It can have an extremely small effect inasmuch as it impacts click-through rate and pogosticking, but there are about 8,000 better things you could do that would make a bigger splash than optimizing meta-descriptions. If this is what your client wants, by all means go for it, but make sure they understand that this will not change their rankings significantly.