First priority should be compensation for both Pellegrino and a new manager (we should not settle for choosing from the unemployed crop, find the right man for the job and ask his club what they want).
Striker top priority after that - i'd be tempted to look at Depoitre from Huddersfield, Assombalonga from Middlesbrough or Antonin Barak from Udinese.
After that I think the only additions we should make are if players go out - Redmond out, another AM in, Cedric/Bertrand out etc.
At centreback I would quite honestly pick Hoedt and Stephens there for the rest of the season. Let them play and progress together.