The 3rd Hokage was Orochimaru teacher, He new of Orochimaru capabilities if The Hokage new of a Jutsu to stop Orochimaru He would have use it before it was to late. He did not had to use the 4th Kokage jutsu and it cost Him His life.
Orochimaru defected from Konoha (even joined the Akatsuki and left). He might have developed succeeding jutsus that Hiruzen haven't seen. In addition, I don't think Hiruzen would have actually known everything and anything about Orochimaru's jutsus. Oro's known to be doing something in the hiding, much like covertly developing jutsus other than kidnapping people for his freak experiments.
As for the "1000 jutsu" thing, whether or not Hiruzen belts a thousand jutsus, I believe he meant it as a hyperbole. It's just a figure of speech. It's silly that one must be required to have 1000 friggin jutsus to become hokage. It's more of one must really, really be powerful enough, and must know and perform S class jutsus.