1. Is this Game Eroge/have hentai scenes?
Book of Zhu: Legacy is not an eroge game and doesn't feature explicit scenes that would qualify.
2. So is this one that focuses on Fanservice/Ecchi?
While some of the characters designs are stylized in a manner that can be described that way, the Visual Novel itself doesn't focus on that aspect. Due to the designs some images may fall under the classification but they aren't the norm and most of the focus isn't on that. Most of the revealing scenes/designs are simply part of the setting's look.
3. Are there different routes?
Yes and No. In any one playthrough you play two interconnected routes, one of which is the central Aya story and the other one of several routes focusing on one of the other characters. This is done to keep a more focused narrative as well as allow the bulk of every path to be canon with only certain details being possibly not.
4. How much will the Visual Novel cost?
While the projects released for Book of Zhu will have varying prices, Book of Zhu: Legacy and it's related content will be completely free. There will be no purchase required and any optional donations given beforehand go purely into making it better and getting it released sooner.