First of all, do all development on a demo server rather than live.
Second, as you mentioned, you should use AIO for seo, also install Google sitemap generator.
If they have a static site, their URLs will probably end in .html or .php – you can set WP up to maintain the same URL structure. That’s probably your best bet. If you want to change the URL structure, be sure to 301 redirect all the old URLs to the new URLs.
If you have any specific questions, go ahead and post them and I’ll try to help you out.