Well first of all, it'd definitely only be Iran targeting Israel. US support of any anit-Iranian coalition of states would be fairly dependent on the assured protection of Israel, so make of that what you will.
As for Russian support; I'm fairly sure their ties are completely economic. Of course this isn't to be discounted, but unlike, say, North Vietnam or the DPRK, there is a fair ideological divide here. If Iran gets stomped, Russia really only faces to lose a business partner, as opposed to any national pride. On the other hand, the have a lot to gain from the destruction of oil drilling and processing facilities across the middle east, so there's that.
I'd put my money on Saudia Arabia. Unlike a lot of other wars, the terms of this conflict's resolution wouldn't be likely to involve any long-term occupation of Iranian territory (except maybe on the water). The war would be fought, the belligerents would get their noses bloodied, and when it ended it ended. Not likely to be a drawn out war of attrition or anything that could lead to situations like Afghanistan or Syria; Objectives are met, treaties are signed, soldiers go home and grudges are held.