Honestly - the best setup for her will likely be to leave her at level 40. You're far too early in the game to be wasting resources leveling her any further right now. She has a few gimmick uses in arena teams that rely on revives - teams with Brakus for example. But beyond that, her utility is seriously limited.

From your screenshot it doesn't look like you have anyone who is level 60 yet. If that's truly the case, I would focus on maxing Elhain so she can help you level others. Technically Aleksander is better, but he's much harder to book.

I agree with kramas here. Also, the problem you're gonna run into with Sachi is maximizing her kit is going to require a bunch of stats.

If you do want to use her: Accuracy, speed, crit rate (for the a3), defensive stats...you could give up a bit in accuracy if you're running Aleks as your lead. That would probably be enough for now. 

Venomage (after your starter, and maybe Aleks too) is going to be a nice 3rd champion to add to your teams. 

Yes. Taking Elhain, Aleks and Venomage to 60, in that order, is great advice. Just focusing exclusively on taking your starter to 60 will very much push your account forward.