you dont need any, you'l be fine with Djourou/Vermaelen/Koscielny/Squillaci for your first year, while Squillaci should probably be sold in the secound to make room for Bartley
Personally as i start i try to get rid of Squillaci on loan and then the next season i'll try and get rid of him so i buy a cb for first season and use Koscienly, Djouoro as backup and play vermalen and the new signing cb
if you have Uruguayan league: Sebastian Coates. other wise you could get chris smalling on loan if they will let you or Antonio Ferdinand as he is fairly cheap. if needed song could play there too
edit: ezequiel Munoz isn't bad either