Dont be silly. Dont get rid of Cech no matter what. If he's 2-3 years left on his contract, keep him. If he happens to ask for a transfer, refuse. You cant go selling one of the best goalies in the game man. The only ones better you can buy are Casillas and Buffon. So keep him. Once you win a trophy or two (and its hard not to with Chelsea) he'll be happy once again.
And I dont know if you need another CB tbh. You've got Ben-Haim and Alex who are both pretty good. Carvalho out for 5 months is a huge blow I suppose. But you've got Essien who can play CB if needed. See if you can recall Khalid from his loan. I dont know if you'll get that option or not. I suppose it wont be too bad if you got another CB but would recommend a youngster. So get Breno. He's already got great stats and can definitely slot in to the team if needed. If you are almost in Jan, Samba will be expensive by now (he's worth more than 10M in my game in Jan!!). So yeah, get Breno. Or if you dont mind spending, look at Garay. Get a scout report on Steven Taylor (Newcastle). If they say he'll be a good signing, get him. He was playing well for me in my last game. And I think he should be available under 15M. But I'd still prefer Breno tbh. You can easily send him on loan next season if you want to.
As for Drogba, depends on how important he is for you. I'm not a big fan of playing a TM so I would personally let him go (ofcourse the price must be right). Also, the reason you shouldnt be too hesitant to let him go is because there are plenty of strikers in the game which can easily replace him. But unfortunately there are very few GK's who can replace Cech.
Keep Cech, sell Drogba!!