Well imo the problem City face is not needing to buy more players, but needing to buy players that will improve their current line up. They have probably the strongest set of players in the PL, but they don't gel together as well as Utd's, Chelsea's or even Arsenal's. And with FFP coming in they can't follow their current tactics of buy every player that comes available and stick with the ones that turn out to be right for the team.
Every team needs that one or two players (think Cantona, Vieira, Keane, Henry, Drogba) that turn a collection of good players able to compete for trophies into dominant title winners. City still need to find that player and, as Ranieri, Benitez and Keegan found out, finding that player isn't always easy.