At the risk of being shot at
Costa is a sensible short to medium term cost effective option. Oliveira is not. Rondon could be. Let us not forget Raul didn't have much of a record when he came here. To me this situation is why Mendes is so involved. It is within his gift to highlight the right players from his client base , if you want a win/win. Giroud was never happening. Origi could happen and he has more strength than Cutrone or Fabio from the few times I have seen him.
So to answer your question Costa , high wages no fee . Origi medium wages , medium fee or Rondon higher wages , smaller fee. As a Mendes client Costa would be a favourite option.