Hello there,
welcome to the game, hope you enjoy it.:)
First, a small correction. The 'coins' as you mentioned in your post, they are called ingots. Let me describe it as ingots, to avoid confusion.

There are coins - a general currency in game which you can get from a lot of places(survival, rich field, nine tail event, convoy and others).
Coupons- which you c*so get from a few places. You can buy stuffs from shop with these.
Ingot - are those that you get only with recharging money. Although they have same value as coupons in shop, ingots are worth more than coupons - for a small spender atleast.

Now, Jonin medal is a good investment in the long run. Gives you coupons back over time and makes your overall gameplay experience better with a few advantages. But, its not too useful to enjoy the game. I'd say don't buy it for now. Play the game for a few days at first, understand some stuffs as you spend time - and then decide later whether you really want or need it.

If you want to spend a small amount, then the monthly pack is the best thing where you should spend it first. After having gaara with the first recharge event, I'd strongly suggest to spend those ingots on monthly pack.

As for the events, I'd say don't spend for them now. In the first few days you wont need most of them, so just grab what you can get for free.

As for now, don't spend on your coupons on anything. Save them till you understand what you's need in the long run and then go after them. Just spend time for fun, and you'd get the grasp of things pretty soon.

As for ninja, Iruka is a pretty good choice at first and can be a solid investment (for nine tail event atlast).

Hope these helps.Feel to to ask if you have more doubts to clear.:)