I really don't think we should go after Whiteside. He's so awesome at shotblocking that it makes him a very good defender overall, but his deficiencies in other areas preclude him from being a DPOY-level defender. Then offensively, he's awful. One of the worst passers of all time, and he just doesn't know how to get involved on that end. Theoretically he could be a good finisher, but to get dunks you have to first get open, and he doesn't know how to do that consistently.
Whiteside could theoretically improve on his weaknesses, but if we're getting Durant, then there are much, much better ways we could use our remaining cap space.