Gah, that sucks but it's not the end of the world. It is possible to repair rust in chassis rails. You will need to have it engineered, but it shouldn't be too much of a headf**k.
That is, of course, if you have no recourse against the person you bought it from. Was it private or thru a dealer? If private then you don't have a leg to stand on (unless he supplied the RWC - you will then have to go after who ever supplied the RWC), if thru a registered dealer then you should be able to get your cash back (even if outside the 30 day warranty period as it is a chronic problem that must have been affecting the car prior to you purchasing it).
GL with it!