I would save them for the next lucky board. The rewards in this one isn't very good unless you spend $400 for Hidan. I am currently saving my coupons for Sailor Sakura when she decides to pop up again. Honestly though the best thing to do with coupons is to get for power, but I have a tendency to * them on ninjas. The best event to most people though is Lucky Board because of you will get the most out of your coupons, plus everytime you pass start you get a gift box that has a chance to drop Hashirama. If I was you I would save them till the event comes back around in a month, but if you can't wait that long use them on power like when/if the cave key rebate comes around, that will increase your power quite a lot. Also if you ever decide to spend a little bit of money on the game I recommend getting the monthly card pack rebate. 300 ingots get you 300 coupons plus 50 coupons a day for 30 days (1500 coupons).

Unless you can get him from an event you will have to get him in Tendo Treasure, which to get him you need to have used 400 scrolls in that treasure overall. And you may not even get him its 50/50 tendo or hashirama. I know you probably really want tendo (myself included), but I would save your scrolls for jinchuuriki treasures or go for 1 or 2 rare in kage. I say this because in China Tendo is a lot easier to get. Pretty sure he goes from a 400 pull to a 200 pull ninja. This may or may not happen, but it's completly your choice whether or not you go for tendo. He is an amazing ninja, but if you are f2p I don't see you getting him anytime soon.

Not positive on this, but the next treasure after Jinchuuriki should be the Reanimated. May be a while before we get that though.

Doubt he will be that cheap, in the recent land of lightning event Shurado and Gakido were 3800 coupons each and they are rares. Tendo is a super rare, so if he is ever in an event he will probably be 10k+ coupons/ingots