Zahovic is a great player, and he is proving it. He's one of the top players in Europe. Of course I think Rui Costa is a god, but your best scenario is to bring in either JVP or Zahovic, JVP being free transfer and keep Rui.
As far as what happened in Porto, he loves that team, and because of his ego he decided that he wanted to tell management where to send him (Valencia)...Management there didn't take to kindly to that, and sold him for the most bucks they could get, which Olympiakos delivered.
But after watching him for many years, I can say that you will not go wrong with him or JVP or even both.