What are some thesis statements that I can speak of concerning the theme "money cannot buy happiness"? I know this theme is oversimplified...

I think that you are on the right track with this.  I would probably point out that there is a hollowness to the characters who possess wealth at the end of the novel.  There is a reason that Nick leaves once and for all, understanding the Jordan Bakers and Buchannans for what they are.  Gatsby's death and life end up proving that while the desire for wealth is an important part of happiness, it cannot define it in totality.  That is to say, the reader fully understands the illusory nature of Gatsby's happiness at the end of the novel.  The desire to amass wealth becomes something that conceals the emptiness and hollowness that pervades all consciousness.  In the end, money cannot buy this away.  The lack of emotional affect of the characters helps to prove this, reason enough for Nick to reject this life style and head back home where money is not valued as heavily in the pursuit of happiness.