Size the implement first, the tractor second. A 6 m disc harrow wants roughly 30 hp per metre of width in heavy soil.
Then apply your agronomic window. If your planting window is 12 working days and you must cover 900 ha, daily hectares dictate drawbar power.
Overbuying shows up as partial-load fuel burn and compaction. Underbuying shows up as missed windows which cost more than diesel ever will.