the only reason orochimaru would kill tsunade is to open a new hokage seat (to open the position for sasuke), but if he were to do that the remaining kages from the other villages would intervene so this scenario is highly unlikely.
orochimaru's goal is to obtain powers to that of a "god" ie. so6p
it has been shown that one needs either senju dna and the pure body of an uchiha or some other variant we don't know about yet.
it's likely that the body must be "good" otherwise he would've killed sasuke in chuunin saga when he believed sasuke couldn't keep up with him.
sasuke has been nurtured to orochimaru's liking and oro is much stronger now because of hashirama's dna.
it is also highly likely orochimaru IS able to take over sasuke's body since he said it prior to being revived (as well as prior to reviving in a zetsu body).
karin may be able to heal tsunade since i don't think orochimaru knows any healing abilities.
i really can't think of any reason why orochimaru would want to save tsunade unless she can provide something for him.
I believe that Orochimaru intends to awaken the Rinnegan within Sasuke. The EMS is a necessary step right before achieving the Rinnegan, but I believe that the combination of Hashirama's cells and undergoing a near death state would cause the eye to awaken. I believe that Sasuke would automatically activate the Rinnegan since his body underwent the ritual for the cursed seal. With the Rinnegan and the jyubi, Orochimaru can be a true immortal being!
I'm sure the Zestu didn't leave any DNA behind lol. I used to think that it was certain Sasuke would get rinnengan but now I don't think he will. Unless the rinnengan has more to offer I'm hoping he don't get it.