What ever you find more convenient.
I would think it depends on the volume. You can also copy and paste from the live website.
As for search engine and rankings, if you are keeping the page URL’s the same, then you should be okay for those pages. If you’re changing the page URL’s then you’ll want to redirect any old URL’s to a new one.
You just don’t want to remove a old page without a redirect. Search engines cache and index URL’s for quite a while, plus users might have bookmarked the old page or shared it on a blog. You don’t want the page just to disappear.
Good luck with the site, don’t hesitate to ask more questions as they come.