Hello,
You'll get lots of advice here as the 2CV do have some chronic issues like rust under the windshield, hinges and other places. Suspension needs to be checked, etc.
However and not wanting to discredit any of our wonderful rededitors, my suggestion is that you buy one of those books that cover buying a 2CV from A to Z. Here is one: https://www.amazon.com/Citroen-2CV-Essential-Buyers-Guide/dp/1845840992
Other thoughts: service history and where the car spent most of its life (humid? Salt?)