Corrosion at the threshold (arguably concealed deficiency)

My Ka is rusted through both door sills. This was not mentioned by the seller in any way! The places have always been covered by the door seals and I have now only discovered it because I wanted to clean under it. In several places it is completely guttered! That is a maliciously concealed lack, right? What can I do there? Under these circumstances, the purchase price of 1700 € already seems to me a bit high. And only with this I mentioned it: The seller is a private person and the delay (an ADAC form) is still before me. It says: The seller guarantees […] that the car during the time it was in his possession did not suffer any accident damage and no other damage […]. I am grateful for every tip!