If Orochimaru is after a fully developed Sharingan he could have killed, or atleast maimed Kakashi and got the Sharingan from him...

As for Orochimaru taking over Sasuke's body, Orochimaru isn't that old dudes

He would have to be about 3rd Hokage's age before he would need that.

The previous posts have raised a question in my mind... if Kakashi has a 3 dot sharingan, AND IF he got it from a dead or dying Uchiha, then something that killed that Uchiha must have been pretty strong, since he would have been fully developed. Could it have been Orochimaru...?