Preface: I don\'t have experience with rules engines, building rules, modeling rules, implementing data structures for rules, or whatnot. Therefore, I don\'t know what I\'m doin
This is a very complex problem that I'm not qualified to answer, but I've seen lots of references to. The fundamental problem is that for games, quests and items and "stats" for various objects can have non-relational dependencies. This thread may help you a lot.
You might want to pick up a couple books on the topic, and look into using LUA as a rules processor.