In my view, TOEFL writing is to test your writing skills and they mark your essay as per their standard format of writing(i.e four para format ;. intro, body (at least two paras) and the conclusion). It doesn't matter whether you agree, disagree or take a moderate stance -
State your opinion in the intro. Then give your first reason as to why you hold that opinion and support it with examples. For example, you mention -
Happiness is higher with higher ability and lower expectations. The poor mostly know their limitations and learn to keep their expectations under reasonable control. The rich, on the other hand, know their ability and as a result allow their expectations to be higher.
Now give a real life example for this argument.
Then give your second reason and support it with an example. Something like... My father always bought vehicles that managed fuel consumption efficiently and never owned a vehicle of his personal choice because he was well aware of his limitations on spending. However, my uncle, a company CEO, was never bothered about such details and always spent lavishly on his desired cars.
Finally, in the conclusion sum up what you said above and re-instate your opinion.
Good Luck with TOEFL!!!!