You should save up your coins to work on your group contribution first. Group skills give quite a lot more power compared to strengthening your equipment. Until you max that out, group contributions comes first. :)

You need to do a little long term planning for what you want to use in the future lineup and prepare for that too. Like saving seal scrolls etc. Sage Naruto is a pretty good ninja to get when Sage treasure unlocks later on, and most ninjas you can get from Seal Treasure can be gained later on in the game. Its all just some information that you would learn and gain from experience and interactions with more experienced players.

Water mains will become / is one of the best healers in the game, with healing tips etc. It isn't that prominent early on but will haunt your opponents if their battle with you drags on.

Early on you will get Haku which barrier boosts Water ninjas by 40%. That is quite a powerful buff early on and can be capitalized on to deal damage even in PvE when you clear plot instances.