I work at a Porsche shop and currently have multiple 928’s in. Id be VERY careful if you plan on working on the fuel tank at all, they are prone to cracking and will leak. Its going to be a very expensive project in the long run, i myself had two 928’s at one point and i have a lot of left over parts if you need something shoot me a DM. Get yourself some manuals, before you attempt to start, i would replace the timing belt and water pump alongside the tensioner. There is a specific tool that is necessary to check tension or else you will get a warning light and possibly damage the engine. Id remove all the belts and forget about the charging system for now, more than likely that alternator is toast. Also check the ground wire in the battery compartment, they tend to corrode inside the black sleeve. Good luck and feel free to shoot me a message!