How to start an Opel Kadett C (1975) after a long period of inactivity?

Hello, I'm getting an Opel Kadett C for €800. It's been sitting in the barn for 30 years and ran perfectly when it was tuned up. The front left fender and the driver's door are partially rusted through, the sills still look good from the outside and so does the rest, the interior needs to…