Try buying lucas Leiva form gremio(?). You can buy him for about 5 mil tops in your second season. i put him straight in reserves and he is now worth 16 mil and hes only 21.

Mata is also another great buy from real madrids reserves. He is worth 13 mil in his 2nd season and he hasnt even hit 20 i think.

Snijder is an exelent player who gets raved about eery week by the media over his exeltent abilty.
Gago is probably the best midfielder in the game after 3 seasons and Ryan Babel is one of the most deadly strikers in the world if you manage to have faith in him.
If you want quick success try buying Kun Aguero but watch out because his price Goes sky high after a season or two.