tap power is completely irrelevant within a minute of playing the game. It's a waste of gold, don't buy it. The heal from it is almost insignificant as well.
After getting Noble hero, the price for ranking up increases by an astronomical amount. Would be great if there was an end game to compensate. Hopefully the devs are currently working on one.
 "Idle Sword"){kind=icon}
 "Hero Simulator"){kind=icon}
 "The Enchanted Cave 2"){kind=icon}