There is no specific du’a to be said at the time of suhur. What is prescribed is to say Bismillah at the beginning and to praise Him when one stops eating, as should be done at every meal.

The Sunnah is to hasten to break the fast, which means breaking fast straight after the sun has set. For more, please see the detailed answer.

Du`a for breaking fast should be made before iftar, at Maghrib, because this combines weakness and humility, and because he is still fasting.