Realistic would be Rooney leaving and being replaced by an attacker of some sort. TFM going out on loan. One midfielder to be the long term replacement for Carrick, who appears to be staying another season since we are currently negotiating a new deal according to a recent report in the Mail. I can't see our net spend being above £100m.
Apart from that, it depends who actually wants to leave: Zlatan seems more likely to stay than go, though still possible he may opt for LA Galaxy, De Gea will leave as soon as Real Madrid offer us sufficient cash but at the moment it doesn't feel like they want him enough to go big. Darmian might want to join Juventus or Milan but his wages would be an issue unless they are desperate to sign him, which I seriously doubt. Anyway, I firmly expect him to stay.
Outside bet would be Mata leaving.
The team needs fine-tuning rather than major changes. Players improving their finishing, the manager understanding how best to use players, the team learning how to be more creative against a deep block defence.
By the way, I can't believe all these posts that assume we will be replacing defenders who have contributed to a defence that has conceded 12 goals in the 22 PL matches since the 0-4 at Chelsea, 24 goals in 31 games through the PL season to date. Defence is about a group of players understanding what each other is going to do in any given situation. Unnecessary changes are simply asking for trouble.