You failed to provide a critical piece of information regarding the type of impeller.  However, as expected, running a small impeller 140 mm in diameter, at only 200 rpm, the power requirements for any type impeller are less than 0.02 hp.  A 1/2 hp motor is more than adequate, even a 1/4 hp motor is sufficient.  The mixing intensity will be gentle, but should be adequate to move the entire contents and avoid excessive splashing.  Since I doubt that the tank will have baffles, you will get a considerable amount of swirling and probably a strong vortex on the surface.  These conditions would not be good for mixing a larger quantity of material, but you have a small volume which will mix quickly.

The answers by this expert are based on the best available interpretation of the information provided.  The consequences of the application of this information are the responsibility of the user.  If clarification is needed, please submit a further question.