If Iran has nuclear capability -- even theater-based nuclear capabilities -- the threat to the entire world is clear. If, as you posit, Iran truly has no friends in the world, they have absolutely no incentive to do anything other than unleash everything they have in the event of a first strike by the West. So, basically, you undertake a first strike against Iran, you are essentially assuring the world that a nuclear conflict, whether miminal or larger, will ensue.
Frankly, I would think that nuclear conflict with Iran would be a best case scenario in the event of a first strike of our own. Somehow, I can't get really jazzed about the idea of a nuclear war of any magnitude. Call me a chicken or whatever other perjorative term you wish -- I hardly think it irrational or unwise to favor efforts that will avoid nuclear conflict. Maybe I'm alone in that and if so, so be it.