If you're in your first season, Nathaniel Clyne is a good signing, mainly for the future, but I find him better than Baptiste and the inept Eardley straight away. Good to have a young player in your team who's only going to get better.
In the positions you're after, Peter Masilela is a top LB who I tried to sign, but he turned me down. Not sure what the work permit situation is with him, though. Worth a look.
At CB, keep hold of Cathcart. I signed Guillermo Burdisso after staying up and he was a monster, to the extent that he got poached by a French club after six months.
I don't use a DM, but I've got Jordon Mutch as a CM and he can play there. He's a really consistent player and a top prospect. You've already got super Keith "Gnashers" Southern who does a decent job in there when I use an anchor man.
Up front, Lacina Traore has been immense for me and the aforementioned Christian Benitez is a bargain on a free in the summer. Again though, you've got DJ Campbell who's bagged 23 and 17 in my last two seasons, playing everywhere from RW, LW, ACM and ST. I might be biased as a big Blackpool fan, but there's plenty of talent in the squad.
I also advise you to bring in a decent GK. Alex Smithies and Juan Pablo Carrizo have never let me down (OK Carrizo has occasionally!) And don't miss out on LW Martin Zeman, who's cheap and unstoppable!
Hope this helps.
for centre back should look at the 20 year old centre back at West Ham United