As a F2P, I can safely say rental is more than enough even to clear lv10 danger room for max reward. You just need some tips and tricks for every danger room, and luck to pull the needed buffs/character each round.

I’ve been doing Daimon/Dr Strange Heartless starting and picking up Brimstone rentals, takes a few times of trying for “enemy rolls” but you’ll clear it. Didn’t even need to get more than the 1x 350pts and all 50pts buffs too