2. A "nuclear deal" where "Iran get's to keep their nukes and we get to deal with it" isn't a "deal", it is a "surrender".
3. Iran is already dedicating their available resources against IS. It's not like sending them a letter is suddenly going to create a division of reserve Qods Force guys for them.
If we give Iran the nuclear greenlight, Saudi Arabia has basically already said that they will immediately begin their own program. Given your last paragraph, you might enjoy spending some time ruminating on the implications of that.