Hi there,
without knowing what or how the site is built its hard to say.
As a test you could create a staging site of the original. Install GP and GPP whilst keeping all the original plugins. Then take a look to see whats missing or broken. Its a quick way of seeing what was theme specific, such as templates or post types, and if there are any plugin incompatibilities.
The very least you will get a handle on what works and what doesn’t. The latter being your list of GP things to do when you start the new site. At this point i would probably start building out a clean site with GP and import the clients content. As long as the whole thing didn’t explode in the first place 🙂