Every transfer is a risk, but out of the three guys mentioned, Dzeko is the least risky. He'd fit right into our tactical system. Neymar and Pastore, though both are talented, would require more adapting. And both are yet to prove themselves.
Pastore has 1 year in Serie A, Neymar has Brasilian league experience. But talent alone doesn't cut it. Dzeko is the most proven of the two, for three years he's played and scored in pretty much every competition there is, both club and country.