First off, German Bundesliga >>>> La liga. Second off, Edin scored with Wolfsburg against Man Utd so following your logic he shouldn't have flopped at City.
Look both Edin adn Aguero are quality players but if not played the way they're used to then yeah, they'll probably flop. Not to mention how a different football style could effect their play jumping from a league to another.
Just give them more money, Marotta had no problems spending 10 million on Pepe. We can't afford to mess this up. What if Abramovic decides to get involved? We'd be fucked. He'd offer Athletico 45 million and Aguero like 10 million.