You should hire a contractor to fix the sagging joists with a beam. Ripping up subfloor is a PITA and just sistering joists won't fix the problem if the joists are resting on something that's sagging. You don't need a hammer drill to remove the flooring or self leveler, you should.just borrow or rent a burke bar. Even glued down stuff will come up with some elbow grease. Put down your own flooring if you want to save $$.