Volland would be great but his opt out is rumored to be €15M and he'd certainly ask for about €3M a year, which would make this something like a €30M deal, if he signs a five year deal.
Leverkusen has also stated their interest and they can pay a bit more, given that they're receiving a nice amount of money from Bayer every year, and should also provide CL football.
Right now I don't know who it will be, especially with everyone and their mother(di Santo, Mehmedi, Seferovic, Drmic, Mitrovic, Embolo) being linked to us.