If the object is a flat, polished mirror; and if the illuminating light is a collimated monochromatic beam, then the reflected beam will be a plane wave. But in reality, every point on a typical object scatters light independently. The reflected light from a typical object can be represented as the coherent superposition of a lot of spherical waves (each centered on a different point on the object's surface), or as the coherent superposition of a lot of plane waves. And there are lots of other mathematical ways to represent the reflected light. But for a typical object, none of the representations will amount to a single plane wave.
The second equation you wrote is not useful except for a single plane wave. Instead, it should represent U2 as the sum of a large number of plane waves, each with their own k and each with their own A.