About your stamina just use it for sweeps now, soon you'll start needing stuff like awakening materials and such which will also need some minor farming. Mostly now just pick a couple ninjas to sweep for frags and the rest for materials to upgrade stuff or just exp.
If you can get 720 coupons in the next two days from all the milestone rewards and such you might wanna pick up Iruka from the current event at 20% discount otherwise just save up till another event with good returns comes along.
Group contribution should only be used in one thing, upgrading your group skills. The first 5 that raise your basic stats are the ones that give the most power in return so I'd focus on those first. Wasting contribution on Lucky Wheel is not worth it unless you already max your skills.
While your group skills are low it's a far better investment to use your coins to donate to your group daily and level them up with that contribution and spending the leftovers to strengthen a bit or spend them for materials and frags on the Black market.
As a Water Main you definitely want 3 star Haku and Tenten for the 9tails event. Hinata and Sasuke are also very popular choices for pvp once they hit 2 and 3 stars respectively.
As for the shops from Arena you might want Kurenai, from Ranked Sai and Kakuzu, from Group Kisame and Survival you want pretty much everyone but Lee. Tobi's great at the moment although will get nerfed at some later point, Kabuto is very useful and Suigetsu is amazing for Water Main 9tails teams.
Just a side note do not expect to have a universal team, different modes in this game call for different kind of teams and builds. So you're gonna be using a different team for PvP, a different one for ranked, a different one for 9tails, another for Strong Approaching(lvl 43+ instance) etc.