I think we only need to get two defenders in Janurary. 5 months should be enough for them to cope and get used to the team.
The defenders i suggest are:
Mexes/Lucio and maybe Oddo/Grygara.
I dont think we need any more options in attack or even midfield (for now). we just need to play Maresca more often as he proved tonight he can play in any midfield position (left, right, behind the attackers, and in Tacchi's role). Olivera should get more time as he seems very promising.
I also suggest keeping Davids. Any team in the world can use him no matter how many players they have.
By next summer, Ferrara, Montero, and Conte would leave. Mexes, Lucio, and Olivera would already be used to the team and they'll compinsate for the loss in numbers.