Was Saruman the White as powerful as Sauron without the ring? Was Sauron afraid of him? Could Saruman kill a Balrog?
Saruman was a Maia of the house of Aule; that makes him one of the beings who existed before the creation of the world (he would have participated in the creation, but not to such an extent as the greater Powers) which in turn makes him pretty damn powerful indeed.
Regarding relative power, you could place him above all of the Elves, Dwarves and Men in his Valinorean incarnation; on Middle-Earth he would have been constrained by the body he had accepted which made him subject to (hypothetical incredibly slow) ageing and to the cares of the world (ref: Unfinished Tales, Istari essay) so Sauron or a Balrog would most likely have been above his capability (in the latter case he may have stood a better chance than Gandalf though), but I don't believe that Tolkien ever made a definitive statement on that matter.
Despite that, one can conjecture that if he had been capable of defeating Sauron, he would have definitely made an attempt to do so. He wanted the Ring, and Sauron was an obstacle to him getting that. Also remember - it took the full power of the White Council to merely drive Sauron (in his Necromancer guise) from Dol Guldur; not to destroy him, just to drive him out. A lot of trouble could have been averted if they had destroyed him, and if they had been capable of doing so it seems most likely that they would have.
He's probably at least as Strong as Gandalf (the White), Galadriel and some of the other higher level magic using characters in Tolkien's lore, Maybe a little more powerful then those two however, if he was viewed as the Leader of the White Council, although probably not as strong as Sauron, definitely not Melkor, at their full power.
I imagine too, once he made his own Ring he gained some additional abilities and enhancements to his powers as well, Still probably wouldn't be strong enough to take on Sauron with his Ring of Power. If he was to acquire the One Ring however, He could have took Sauron's place as Dark lord, which was his goal. I think, prior to his fall to madness, he was probably the most powerful person in Middle Earth during that period.
I know it mentioned Sauron feared his Voice. however I don't think he himself feared it as much as he feared Saruman's abilites with it, possibly Sarumon could turn his army against him, or a portion of it if need be. And I'm sure that he could kill a Balrog. If Gandalf was able to and he wasn't as powerful as he would become, nor did he have his staff, I'm sure Saruman could make quick work of the Balrog.
As the Valaquenta says:
Sauron [...] was only less evil than his master in that for long he served another and not himself.
We have also seen him raising mountains around Mordor. So said Galdor in the Council of Elrond:
And yet we see that Sauron can torture and destroy the very hills.
And I am reasonably sure there was a quote of Sauron raising the hills around Mordor to protect it but I can't find it. And that is a vala-level skill indeed.
Even if not weakened as the istari were, a maia could not have confronted him alone. (Alone -- Sauron did bow before Eönwë but he had the Host Of Valinor at his back and while it's not clear where Tulkas was at the point, it's quite likely he was not far and as we know he was stronger than Morgoth himself in battle.)