He’s not interested in a body that can run around, he can go out into an animal for that. He needed one that was compatible with the Great Weirwood tree. His mind will be out and about but he apparently needs a beating heart in order to maintain individuality; otherwise he will be completely subsumed by the forest, animal, other trees, CotF, and so forth. Dying is like joining the Borg, but if he has a body he can think separately. The CotF mentioned that he was coming to the end of his magically extended life.
He also needed someone who Jojen, the CotF, or folks like them would be willing to bring to him. Someone they would recognize as exceptional and be loyal enough to assist at the probable cost of their own lives.