You can use a 1 hp motor at 2,800 rpm to drive a mixer at 2,000 rpm, provided you have some type of speed control.  The maximum size impeller you can use with a 1 hp motor at 2,000 rpm is about 100 mm in diameter.  Larger impellers cannot go to 2,000 rpm and stay below 1 hp.  Although I doubt that you will keep the oil in your vessel at those speeds anyway.  Lower speeds are always an option.
 
If you center mount your mixer, you will definitely need baffles. You may be able to angle, off-set your mixer with a hydrofoil or propeller and still mix, but probably only with a small impeller, less than about 60 mm in diameter, and then perhaps not at 2,000 rpm.
 
The biggest factor with respect to mixing intensity and power is the impeller type and size.  You can have any size motor you want, but the only power delivered to the fluid is determined by the impeller type, impeller diameter, and rotational speed.
 
Torque is easy, it is power divided by 2 pi times rotational speed:

    1 hp = 746 W    2000 rpm = 33.33 rps
    Torque is 746 W / (2 pi 33.3 rps) = 3.56 N m


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