You could demand that the vendor puts it though a complete VIC test, though this is not a legal requirement for a Cat D. More sensible to ascertain exactly why it was a Cat D write off. May be no more than the car was stolen and paid out as a total loss, then later recovered with no damage or only minor damage. The most likely damage on a Cat D is to the locks and immobiliser, and may be 'damage' to the software. You need to perform an HPI/Experian/RAC history check to make sure it is not still recorded as stolen or with an HP debt or a chattel mortgage against it.