IMO we don't need to invest anything in GK, defence and attack.
Midfield should be priority. We need 2 players there, since Sturaro and Marchisio don't seem to be in Allegri's plans. Also Khedira and Matuidi on the field at the same time make us weak.
I have a feeling we will try for Milinkovic and fail. So we will have to buy someone from Roma or England. In the end there's always Cristante.
In attack, we need another dribbler who can break open defenses and counterattacking option besides Douglas Costa. This is very important.
In defense, if Cuadra is our RB next season

tup

, need a conservative/defensively solid RB as his backup/alternative. A new Howedes if you will.
Biggest priority is midfield, yes. 2nd arrival after Can hopefully will be SMS/Pogba/Di Maria, but unfortunately, Bonaventura looks more likely.