The Manager shall, by May 15 of each year, submit to the Council a budget and an explanatory budget message. For such purpose, at such date as he shall determine, he, or an officer designated by him, shall obtain from the head of each office, department or agency estimates of revenue and expenditures of that office, department or agency, detailed by organization units and character and object of expenditure, and such other supporting data as he may request; together with an estimate of all capital projects pending or which such department head believes should be undertaken (a) within the succeeding fiscal year and (b) within the five (5) next succeeding years. In preparing the budget, the Manager shall review the estimates, shall hold hearings thereon and may revise the estimates as he may deem advisable.