Brother, dont tell me you are joking considering 20% consecutive users of ~1000 estimated and putting them on a shared server? I am telling you right now, even 5 people taking quiz at the same time on a shared server will pull down the whole shared server, if you are on Cloud Linux, then it will be only you getting effected.
How much is you Budget anyway? as to speak about forum posts and quizzes, you are talking about constant database connections.