I believe most Humble Bundle products are available outside of their bundle windows but at a higher price elsewhere, so most Humble Bundle offerings should be fine. I wouldn't be concerned about it.
There are no truly required aspects for visual novels, as odd as that sounds. We've had accepted entries before that do not even have text and instead use emoji exclusively to convey a story. Voice acting is something we are actively encouraging developers to pursue, but it is not required. Many previous years winners did not have voice acting in their submissions at all.
Let me know if you have any more questions!