Your OpAmp load impedance will be less .... than, how much? Negative feedback around the OpAmp decreases output impedance of the OpAmp, I am not sure it is not enough for you to drive the quasi resistive mixer input impedance with a common source amplifier. You still have to charge and discharge a capacitor in the mixer, which is 1pF, I believe that a class-A OpAmp should handle that if your closed loop gain is maximum 6dB, and you require ~20MHz bandwidth. However, a design of a class-AB common-drain amplifier, or a Monticelli tybe class-AB have many advantages, obviously.