hi i am thinkin about buying a grand vitara 1.6 petrol

I am wondering about a grand vitara petrol 1.6 litre i was going to buy he said there is a knocking when the engine goes over 3000 revs but goes after engine goes up gear he says to me it is a cat convertor problem exhaust and is a cheap fix as in a couple of hundred if you could shed some light on this i would be grateful