Hey.
1. Karin is mostly for stacking poisons, which could eat through shield; she also gets shield from Flower Guard. But she falls out pretty fast in later game. As for Sai... you really don't need him all that much, aside SA as extra damage for Shizune/Karin. If you need a healer, I suggest Karin and Sai's lovechild Kabuto: Double healing/cleansing + clone-spamming right there.
2. Any version of Kankuro is fine - he's an ok combo starter in early game. His puppet could go pass Hinata/Kimimaro's defense, and your Sasuke (if you plan to * with him) could combo into Repulse and Hinata for Accu. His puppet is very thin though. If you plan to use him, 4 star him asap.
3. I still use Kureinai on regular basis in Survival - she heals ANY unit in you team, puppets, summons and clones included; plus, her Genjutsu Mirror comes in handy when everybody (in Survival) is running a variation of control strategy. Boost her resistance if you use her to get maximum effect. But she's a support nin though, so don't expect much damage out of her.