I disagree about Boateng, have you not seen him this seaon hes been imense, hes actually added pace into our midfield...something not seen for many years, hes actually fromed into one of the best CM in Europe as for Nesta hes still amazing, yeah he may not have the pace but his timing and movement are still brilliant, only Silva, Lucio and maybe Vidic are ahead of him, he proved that especially in the games against Barca (1st leg mainly) where he was amazing.
Also Abate, I think hes a top RB although he hasnt done anything brilliant this season, hes been decent, I see no reason to want him off the team. And Robinho its tough on one hand on his day hes a World beater but when hes having an off day theres not many players worse, I wouldnt say hed be a sub at every top team, at Man Utd, Chelsea, Juventus, Inter, Arsenal id say he would be on the 1st team. Hes talented especailly if he played as well as he did last season id defo have him on the first team even when Cassano is back.
As for signing Hummels, Jovetic, Chielini, Maggio and Hamsik well we have to be more realistic, 1: replacing half the team and expecting them to jell and play well together is extreamly difficult and 2: I highly doubt that Napoli would be willing to sell 2 of the best players including the star man to another Serie A side and Juventus wouldnt sell Chelini to there main rivals.
Signings we should make are Handanovic (I agree with that), P.Lahm and M.Sissoko, they would fill the 3 gaps in our team.