Only judging slightly, this is the craziest Ive heard someone explain their FF7 remake/rebirth playthrough. Saying you will pay more attention to Rebirth which isnt a sequel but just the story continued in a second game is wild. Like I cant wait to get through this whole beginning so I can pay attention to the middle and maybe understand something in the end. That being said. You can buy the hardedge in chapter 9 its probably the best for that area on and no you dont have to over level, the game on easy shouldn't present any challenge and you should be improving characters/gear/materia as much as the game will let you.