2nd Storey subfloor is super wavy. Why?
no builder in his right mind would have installed hardwood on a wavy floor.  it sounds more to me like their was a flood which has damaged the hardwood. its swelling, subsequent twisting and then drying has caused the randomly warped floor, but it could also be too high of an ambient humidity, which can be a sign of a bad vapour barrier system and all the nightmares that come with that.

just get a certified engineer to write it up, or even better, just run away as fast as you can (unless you are getting some ridiculously good deal on it).  remember, many builders are shysters at heart, so other things could be wrong that are currently hidden.