I think at first he just wanted the Sharingan to learn every jutsus he could with them but since he already switch bodies to try and stay alive, he figured why not just take the Uchiha's body which can handle the Sharingan better anyways. He first went after Itachi and got beat so went after Sasuje's body when he sensed more power in Sasuke's eyes than Itachi's so from there he wanted only Sasuke's.
Orochimaru needs a strong body in order for him not having to keep switching bodies every 3 years, so if Sasuke's body is strong enough, which Orochimaru believe it is, then he could live in that body for 30+ years until he wants a younger or better body again. The whole SM thing is wrong though, SM enhances your body so why would you need a strong body when it already enhances it? Orochimaru tried copying Juugo's clan ability into another body but couldn't find one that could handle it; he pretty much tried to do what Kabuto did with Karin, Suigetsu, Orochimaru, and the sound 5 abilities and put them in himself.
Having a strong body for SM wasn't an issue when Naruto or Jiraiya learning it and snakes/Dragon SM doesn't look to give as much enhancements when you compare them.