Don't need a new keeper until the end of the first, so I'd wait until then. De Gea is ok, but I'd personally buy Neuyer.
Giggs replacement -
Ashley Young
Eden Hazard
Scholes replacement -
Wesley Sneijder
Pastore
Sneidjer is the more exspensive. He cost me 40m in my current save and hasn't played well. First season in the league though, so will have to wait and see.
Carrrick replacement -
Andrea Poli - Costly though, and plenty of simiar type players go for much cheaper.
Berbatov is a strange one. He isn't the most prolific of goalscorers, but he has the ability to supply a lot of goals. Personally I believe United have the strongest attack in the game. Welbeck and Hernandez both score loads as poachers, and Macheda makes a good complete forward after a bit of training.