The question contains about half the information necessary for a specific answer, but reading into the question several important assumptions, I offer the following:

First, 300-mm diameter pitched-blade turbines in a 2,000-mm diameter tank are about as small as practical. Second, the 250 cp has almost no effect on the power and mixing requirements, however, I have assumed that the density is 1,500 kg/m3. Rotating the mixer at 5.83 rps will require at least a 1,500 Watt motor, but a 2,250 W motor would be better.

Answer: 40 N m torque — higher torque at higher rotational speed, lower torque at lower speed. The lower speed may not mix well.