For my Zeta I will get CLS IBAT anf Han Solo. Should I get the R2 one before Palpatine or ?
It's a really, really close call. If you're using CLS as your arena team, I think I would do R2 first.
If you're not using CLS as your arena team, do Palp first, but R2 right after.
Also, from what I can understand I shouldn't get the Old Ben Devoted protector just yet for my jedi team?
Yep. It's not that it's a bad zeta, there are just so many better ones. Bossk's leadership is better and pulls together your entire Bounty Hunter team. To get C3p0 you're going to need to zeta at least 2 ewoks, and then you have to drop a couple zetas on C3p0 that are both way better than Old Ben. Vader's unique makes him a much better ally for Palp and improves his ship at the same time, since anything that improves the pilot's GP scales up the ship stats (especially health). The night sisters need at least 2, but preferably 4 zetas to come together, but when they do, they're awesome.
Old Ben is a decent toon, and his zeta is decent, but using a zeta there just holds you back from doing even better things elsewhere. (Even if you're strictly focussed on Rebels, you can give the zeta to an ewok, get your C3p0 that much faster, and c3p0 makes your rebel squad ***so*** much better.)