Full back should be our No.1 priority imo, Clyne and Shaw won't play every game and we need someone capable of coming in and doing a good job. Fox is nowhere near PL quality, Targett hasn't been taken to Spain so can't be that close to being ready for the first team, and the likes of Cork, Maya, Chambers would all be playing out of position.
I don't think we necessarily need to spend millions, and although acting as cover, whoever came in would still get plenty of game time. We don't seem too worried about overloading in midfield so I'm not sure why this would be a problem at full back. It is a squad game these days.