I'm very skeptical of whether or not players like Gundogan or Kroos would do better than if we played say, Rafinha there. .
Kroos is better, the problem with youth is that they need a few more years before they are world class (if they ever reach that level).
Kroos is already there, intelligent, good technique, comfortable in possession/passing and cheap because his contract will expire soon (15).
a midfield with busi, cesc, iniesta, kroos and sergi + Youth/xavi would be absolutely fine to win trophies.
not every canteraplayer is world class. Montoya eg will never be absolute world class but he can become a very valuable back up for Barca during the rest of his career,
you should be careful when you buy and sell, too much players/changes can be a problem when you have to adapt all these guys
Buy what you need which are 2 GK, a CB and a proper striker. a CM optionally imho,