my choices: Mexes for sure, and then either Van Buyten, Ferrari or Bonera. I'm leaning towards Ferrari because firstly, he's a great leader and defender IMO, and secondly, one of Van Buyten's strongest aspects is his height, and we already have plenty of that in Tudor and Legrottaglie. Bonera would be a good option as well and his versatility (he can play right back) would be useful, but as far as the CB position goes, Ferrari offers a little bit more from my view point.