Why does Wrath want Ed's limbs?
Wrath's goal was to be more human, like the other Homunculi. However with Wrath, he wanted to take Ed's life, in other words making everything that Ed owns his own. He already had two limbs that Ed "gave up" so why not take the rest?

I'm quite sure he even says that he wants to take "everything" from Ed during their fight under Liore when Sloth and Wrath appear (where Ed first discovers Sloth is his mother and we find out Wrath's weakness).

This can also be seen during the fight against Sloth in the weapons factory. Wrath takes the box in which Ed stored bone of his mother's and fused it into himself, taking something Ed owned for himself.