I'm not sure what was wrong with Cahill / Samba for Arsenal, both offer something a little different. Cahill doesn't have long left on his contract and Samba plays for a *** side. They need some balls in defence, someone to take charge and organise them better, a mouth.
A holding midfielder is more important for Arsenal (imho) then a creative midfielder, You've got Wilshere and Ramsey that are very good, Rosicky and Song could play in midfield too. Frimp being more back up / rotation for the holding midfielder. Ideally an experienced one, but a good holding defensive midfielder is a must still imho.
A left back and a back up right back are very important too and I still think they need another striker, Dzeko would have being an amazing purchase. Robin VP is unable to keep fit/injury free, Arsenal will also lose some of the forwards to the African Cup of Nations, so a striker coming in (or holding on to Bender and actually playing him as a striker) would be a good move too imho.