This is straight Joel Rosenberg fiction becomes reality.
The Saudis do have a lot of resources, but it does take time.
Their best bet is to build a secret assembly line, as well as underground coastal facilities, produce and station thousands of short and medium range conventional rockets and missiles along their coast.
Thus, if Iran carried out any major act of belligerence, it would get an unexpected pounding, so fast and hard that it would seriously foul up their war plans.
Doing it this way, the regular Saudi military would then be able to counter any remaining Iranian counter attack.