yeah, you're freezing at 70 and potions will provide a great increase in power; just don't buy them if your assist rank is already really high; i'd much rather focus on other simpler neglected powerups eg tactics/summons/battle armours. i'd go for the potions as they're high in value but just remember that's the only thing you'llbe getting (and that odd seal scrol/even edo minato if ur truly lucky :) ) so take the initiative and only do what you feel is right