As a f2p Ninetails is something you should prioritise. Not yet for u but soon when your server can kill him, the rewards is huge. For me it's normally around 1m coin, ~450k exp and at least 25 coupon per ninetails.
Ninetails start at 20:10 server time on mon, tue, thu, fri, sun.

For now you only need to buy material and fragments of ninjas you really want. But later on when your server can kill kyuubi you will have more coin to spare. Try buy all ninja fragment from BM except for konoha genin or ninja that have stronger version released in game.