To be successful on a consistent basis is that you need to have a good blend of experience and youth.
We look to a larger scale say over 3-5 seasons or so and invest base on that time scale, where as other clubs just look to the present we look to the future, which is why we invest in players like Smalling, Jones, Powell etc.
Generally we look to younger players in the transfer windows, if they are experienced then they are usually well established in the PL. At the minute we can see that SAF is building for the future, investments in younger players Smalling, Jones, Hernandez, Powell, Kagawa etc coupled with one or two established players like Young. We are still in a transitional period in my opinion, we have a great range of ages at the minute the youth: Welbeck, Smalling, Jones, Cleverley Experienced: Ferdinand, Vidic, Evra, Giggs, Scholes, Park, Carrick coupled with the mid experienced players like Rooney, Nani, Valencia, Young, Evans these are the players we should be building around.
In terms of signings it depends on what type of player we are looking for, in my opinion creativity isn't the problem so personally I would look at more a defensive sided box to box midfielder, in terms of that position we have Carrick and Fletcher who are both well experienced so investing in a younger player 22-25 age range would be better suited so someone like Martinez or Strootman would be a better investment.
If we are looking for a creative mid we have Scholes, Giggs for experience and Cleverley, Anderson, Powell on the younger side so I would say we are fine in this area but looking at Giggs and Scholes it will probably be their last season so someone with more experience to settle them in before such players leave could be a good idea and a Modric or Cabaye would be more ideal than a younger player.