Several serious failures of logic in that OP.
MUFC_BENGI:City have two - Silva and Aguero.
If Silva and Aguero are considered 'world class' then both Nani and Valencia should be too. And Rooney is significantly better than either. Silva is just a playmaker, Aguero just a striker (albeit an impressively flexible one), but Rooney is both at the same time.
MUFC_BENGI:A central player like Xavi, Iniesta, Ronaldo,Robben, Messi.
Ronaldo and Robben are not central players. Ronaldo would best be described as a 'wide forward'. Yes, he often cuts in, but his starting position is still always on the wing. And Robben is a straightforward winger. Even Iniesta is only halfway to being a central player, preferring to push wide.
I'm not totally against buying in a really world class player if the right one is available for the right price. And you're correct that the place for a player like this, at the moment, is central midfield, because our wingers, strikers and defenders all look fantastic. Neither Sneijder and Gotze is right for the position, and buying someone only to adapt them to an unfamiliar position would be a foolish way to go. Like I said, only the right player for the right price.
For me, Modric is really the perfect option, and indeed the only truly suitable player available right now. Given our quality and depth in almost every other position, I'd be willing to see us spend whatever money Ferguson is given over the summer to tempt him away from a wobbling and potentially Harry-less Spurs.
However, if you wanted to buy in a youngster to become Rooney's understudy and eventual replacement in that peculiar false number 10 position he plays (and which no other member of our squad can quite fulfil), look no further than Mateo Kovacic. 17 and already a league winner and Champions League starter against the likes of Real Madrid. A game in which he looked very much the equal of some of those galacticos. Mark my words, this kid is going to be something very, very special indeed.