Measureing reactive torque using Gear motor or water cooled air braking system

I am building a machine to measure the torque out put of a Positive displacement motor powered by water pumped using a triplex pump ( a dynometer). The triplex pump is powered by a 100hp electric motor , the fluid is pumped at Max 150gpm and at 150gpm rpm is @ 375 and will produce @max 1500psi of system pressure .When full resistence is applied to the PDM useing the air brake the reactive torque measures @ 1400ft torque and rpm slows to @ 70 right before stall but can spike to @ 1800 ft. Currently I am using a water cooled air brake to provide the resistence that is to measures the load on the PDM but I want to switch to a hydraulic gear motor . Can any one tell me how I can chose the right gear motor ??? how much pressure it may produce while load is applied and what flow rate the oil pump should handle??