1 Depends on how powerful the attacker is and how you have placed defensive/offensive modules.
2 No
3 Enough to defend it against the current technology level of your opponents if you think it will be a target. At least put on the basic Attack module so you can put on defensive stuff. Even if the mod is not effective against one type of weapon, it will still add at least 1 to the defensive roll total. (Actually it adds the square of the value).
ex. Defending against a laser atack, 9 armor + 9 PD + 9 Shield = 3 + 3 + 9 = 15 defense total with all 3 typesof modules; Just the shield module will equal 0 armor + 0 pd + 9 shield = 9 defense total.
Add as much attack modules as the opponents defensive technology requires to be effective. Room is not an issue, only your ability to make Constructors. There is room for all modules on each type starbase.
Lots of tip on the Gameplay forums, Strategy Forums, in the Wiki, and in the Databanks sections.