Yep, I started as Sunderland as well, and blimey it is a small squad! But anyway back to your question.
In CM look to buy Kevin Thomson from rangers at a bargain price of £1.5million. He's on the transfer list so he shouldn't have any problem bringing him in. From how he was played for me, he has been consistent and the occasional goal threat as well from CM. He has fitted in well with Cattermole and Cana.
At RB I bought Tony Hibbert from Everton for £2-3 million in the end. He hasn't played for me yet due to injury, but fingers crossed he will perform.
At LB I tried to sign Jelle Van Damme from Anderlecht, but I didn't have enough funds as I had spent money on Milevyski instead. From Fm 2009 experience though Van Damme was a good consistent player, and extremely versatile being able to play at LB, CB, LM, CM, and DMC. However his best positions are LB and CB.
I wouldn't sell too many players unless you are really confident of replacing them with better ones, as your squad is small enough already. I didn't sell any in the end, though I did try to sell Kenwyne Jones - not that I disliked him, but - because i didn't think I would be able to sell either Bent or Campbell as they had both just joined the club.
Hope this helps and good luck.