Lucky Ninja Board is pretty good source for power early on, especially if you spend the points on stuff to boost power instead of ninja which is what I do. Cave also decent place to spend during the cave rebate event. Main thing you want to work on with boosting power is refines, threads, cave, 8gate for initiative. I wouldnt spend coupon on anything that don't help with initiative. Other sources of power you will slowly build up along the way.
Also SA hard is possible to do for any level with the right team setup, just google search SA hard. Like a pretty easy team to beat hidan's SA hard is Kimimaro, Kabuto, Tatuya. There are lots of variants though, just got to search around.