CB - Jonas Ivens - FC Groningen 170k release clause, Jores Okore 1.3M, Erik Sviatchenko 1M
CM - Saints I think have a good midfeild already but if you want to bulk up a little then maybe take a look at Adryan - 875k release clause, simao - Shandong and at end of 1st season on a free Vladimir Weiss hes a winger which you didn't ask for but for free hes a bargin!
Hope this helps a little
I second What LukeGame said, and, to make the list even shorter: CB - Jores Okore. CM - You really don't need anyone, if you avoid serious injuries. First choices should be: Schneiderlin for defensive/box2box duties, Cork for box2box duties and Ramirez for AMC (all can play as playmakers).
I would maybe get a DMC. You will be underdog in most games in the PL, and Schneiderlin is the only one in the squad who can take that role. And he's more of a defensive playmaker, than a ballwinner/tackler.
Maybe have a look at Even Hovland (Molde, Norway), who is a CB and DMC + a decent squadplayer for CM.