My issue with buying a reasonably old CB is how quickly will they adapt to a new system? It's not like they'd have time on their side, and given the Barca style is fairly different to anywhere else, you don't want countless mistakes in the middle.
You look at the likes of Abidal, Chyg, Thuram, Adriano, even Maxwell at the start, all struggled when they first came in. I'd say only Alves and Milito (both already in La Liga) slotted in comfortably. So bringing in a Mexes for example could be a huge mistake IMO.