I think it's a mistake to think "we're weak at ___ position so we should target players who play there" in today's NBA. I think once Aldridge left, we became a team of interchangeable parts. What we do need to think is "we do ___ poorly as a team, will x player help us with that?" So maybe that means we target players like (possibly) Whiteside or Biyombo or (more likely) Ezeli, but not because we have a hole at C, and that shouldn't make us think "so we have to get rid of Plumlee now". If Toronto re-signs Biyombo are they going to trade Valanciunas? Only if he's unhappy or they're financially strapped, but not for basketball reasons.
So the kind of players we need to target are versatile, team-oriented and unselfish. Oh, and good defenders.
For the reasons above, I also think we're okay at PF. Aminu defends most of them (and is better suited to PF than SF because of his shaky dribble), Ed can rebound like a mofo, and if they're really big, Plumlee can defend them. Plus, of course, the coaching staff might have more belief that Vonleh will develop than some of us (me included). Now, none of them is a star, but who cares? Each of them can be part of a really good unit on the floor.