Recall Paulbee that Kabuto said his Edo Tensei is a VARIATION on the original jutsu. Clearly he's made changes/improvements, allowing him to achieve stuff Orochimaru probably never considered.
And that likely includes the realization of Edo Tensei's limited "reach" (only souls from the Pure World).
I'm betting Orochimaru prepared three bodies for Edo Tensei fully expecting the 4th to appear, given he didn't know Minato's soul couldn't be accessed. In fact, Orochimaru was surprised when his summons failed:
http://ani-haven.net/hr-alpha/Naruto/118/20