Daniele De Rossi
Well... you obviously need a playmaker and a BWM... so, we'll start right there.
Nigel de Jong is an excellent BWM. The problem is that he plays for Manchester City. Fortunately, no one at City has any loyalty, so getting him to sign a contract with United should be absolutely no trouble at all. The price also isn't astronomical, as he is normally transfer listed after one season...
There is also Marouane Fellaini of Everton, but then you have the Everton-United loyalty problem.
Probably the best choice in my opininon is Radja Nainggolan of Cagliari. He now plays for Liverpool, as in, I just heard of it now, but in the game, he's still in Italy, so there should be no trouble at all getting him.
Realistic playmakers are of course Shinji Kagawa although, he's more of a winger, Luka Modrić (Tottenham), Yoann Gourcuff (Lyon), Toni Kroos (Bayern München), Siem de Jong (Ajax), and perhaps, the long shot of Jack Wilshere... but I prefer him to play as a DLP rather than an AP position... so you can use Yann M'Vila of Rennes.
You could try to get Marek Hamsik of Napoli(playing him as central midfielder-attacking mentality- will make him score lots of goals.) There's a little catch though, you will need a VERY GOOD bwm if you are playing 4-4-2 or an anchor man like Fellaini. I could suggest Kara of Tromso but he is rather for mid-profile teams i think, like Palermo or Q.P.R.
Douglas Costa is amazing if you play 4-2-3-1 .. Amazing player.. can score wonder goals.. but sometimes frustrating.. but amazing Young player.. and there is M'villa..