2 gallons of water in a foot well with windows up for a period of time will destroy electrical under dash and instrument panel etc. due to high humidity and condensation not to mention the moldy interior. That alone would be enough to kill a 928 if the rest of it isn't mint. The biggest reason 928s suffer from failures is due to electrical gremlins, anyone ever seen the factory relay box in one of those???. Good lord, what a debacle. The distorted door panels should be a dead give away this car saw serious water as in at least higher than the door bottoms.
For reference to overall value-
1978 928 porsche
If anyone wants to see what a factory fuse/relay panel looks like in a 928 then here you go- if it weren't for this I would pick a 928 up for kicks.
Troubleshooting my fuse box 1986 928S - YouTube