I play a 4-2-3-1

it has work for me very well. There are some players that you might want to look at like Isco, Lamela.

Lamela is probably one of the best winger in the game. You can get him at around 20m or less. As for Isco he is really class AMC.

Also around the 20m price range. I wouldn't reccomend you get another defender or goalkeeper since you already have what you need at the start. If you want another backup defender, Jores okores is enough since he can play anywhere on the defender. As for CM strengthening, you might want to look at people like Ikkay Gundogan or Fellaini. For striker, just get a cheap backup. Giroud and walcott should be enough for the time being though honestly walcott is much much better as a winger. Giroud is a real beast as a striker.

use him properly and he can give you 50 goals per season XD Your main part to strengthen is actually your midfield. The others aren't that pressing. Koscielny is already a world class LB at the start of the game. Even though thats not his preferred position.