Good choice so far, I think you might want to consider few of the following: Srna, Shaqiri, Witsel, Alvaro Pereira [as you wanted Porto player], Andre Santos, Ayew, Mbia and Smalling.
These are the low cost ones, obviously you've done very well upfront, I think adding Shaqiri in midfield will be good. Hes suspended for match-day one but hes very low cost and likely to get a few goals/assists throughout the other five games. Good to have off the bench. Another of these reliable players would be Srna, takes PKs, FKs and provides assists at Shakhtar.