I've played the game for more than 2 years, read the manga and watched the anime. Uhm, I think I know the world pretty well to manage a game for OVA ^_^
However the original work is the manga, then the game and last the anime.
What are you asking it's a pretty big project... and I've not much free time to think about it , but I'm glad to help if you start to post some guidelines and ideas for the classes.
Thanks in advance for anyhelp you could give me, I really look foreward to seeing what you come up with. Classes wern't something I was even going to approach, I was mainly using the setting as a background.
The greatest evil of all is the indifference of good men.