The lack of central midfielders is why we fell down the table last season, IMO. We have quality players but after an injury or two, our team looks shaky.
According to reports, his wage demand is the reason why Arsenal are struggling to sign him. If that's the case, I'd say we go after someone like Bender or Inler, as you said. A centreback is a need as well but unless we're going after someone young like Doria, Balanta, etc, I don't know who we could get. PSG paying £40 million for Luiz has set the precedent now.