im lookin at buying a car.the car seems mint,the bloke bought it with a blown engine.

ive done a hpi check and it says cat d write off.
told the owner and he was very shocked as he hadnt done a check when he bought it.
now when he bought it the engine was blown and all he can think of is that it was registered cat d cos the owner must have claimed on insurance and cost of engine was more than value of car etc.
could this be correct? any thoughts???