Keep saving your coupons. It's better to wait for the Lucky Board event, you get items of more value than you spent in there.

Getting a full set of lv2 mangatama isn't a bad idea. Ofc, you overpay for this but early on such boost can cement your position in the top bracket of pvp areas. As for lucky stars- you cant use either medium refines or myoboku yet, so skip that event. Like person above me said, best save for next rotation of lucky board or mission dealer and spend there, so you can both get some power and start piecing your future team.