No, it's not that. Both "afim" and "a fim" exist in the language, but they have different meanings. "Afim" means something is similar to another thing. "A Fim" means that something is willing to do some other thing. Different meanings, different writing, same sound.
"Não tenho vontade" is something that lasts longer. For example, "Não tenho vontade de ter filhos". It means I don't want to have children. "Não estar com vontade" is temporary (Gosto de futebol, mas não estou com vontade de jogar hoje).