Just took a shower and gave this a thought. I come up with this answer. man United is top top club as good as any other in the world in any area but transfers. Making transfers i would say United are 3rd rate team.
I would say the 1st rate transfer teams are Real Madrid and Barcelona. They can pretty much get any player they want. They can outbid any club in the world, they can offer bigger wages when any other club and they are more attractive than all the rest clubs
The 2nd tier probably is Chelsea and City. They aint as attractive destination as the Spanish Giants. But they can outbid and offer bigger wages than all other remaining clubs. Which make those 2 the 3rd and 4th most wanted destination for a regular player. There are some players of course like berbatov who turned down City to join United. But i guess these a minority cases.
Finally there are third class clubs in transfers: United, Inter, Milan, Bayern...
I think considering this, it is easy to clarify what is realistic target for United. It is any player that Neither Barcelona, Real, City or Chelsea may be interested in. Also not a player who plays at Inter, Milan or bayern (or Local EPL rival like Liverpool, Arsenal). Because United may not be willing to pay over the top to get a player of those big clubs. But Outside those teams. I am sure United can get any player they want.
Oh and one other factor. As Real and Barca can get pretty much any player they wish. They tend to sign player just for sake of it. And often top quality players are ejected to make a room for new arrivals. I dont think those could consider realistic targets for United either. United seem to have some sort of pride not to take rejects of the barca or real. Probably Because United consider them self just as good the those clubs. But because of that United miss on some bargains. Looking in the past few years clubs like Milan, Inter and Bayern without such a pride that United have managed to obtain top stars like Ronaldinho, Zlatan, Eto'o, Sneijder, Robben. This year there are rumours that Real will eject Diarra, Kaka, Higuain. I wouldnt consider them realistic targets. Not because United wouldnt be number 1 destination place for those players. But because we wouldnt take the Real trash.