Hi Divit,
Monsoon is a tricky season to trek in the hills, so I suggest you to inquire about the weather right before you leave, as landslides are frequent in Himachal.In Old Manali you'll find a lot of shops that can rent you gears if needed, although you might not need a lot of gears for this trek. I'm attaching a couple of articles for you to get a better insight on the trek.