I've watched an expert rebuild half a dozen 928 transmissions, it takes an experienced eye to know which parts need replacing, but its a cinch its going to be more than the synchro, dog teeth go fast once the synchros wear much. Talk to
928 International | Porsche 928 Parts for Sale Online - New, Used & Rebuilt or 928srus to get a better idea of needed parts.
Sitting 15 years chances are very high the old coolant has eaten away at various parts of the engine and heads, no way to tell without pulling the heads.
Any special reason for picking this project car?