I have been tempted to buy Cat D cars in the past as like its previously been mention they are normaly dirt cheap compared to what they would cost.
I just never went ahead and bought one as I didn't like the fact its been a right-off and who knows how well its been fixed. I always thought that the insurance would be so much more as well when insuring a Cat-D, as surely they won't give you the same price as a car that's not been in a right-off.
The question of whether they have been repaired correctly is a separate issue as
ANY vehicle that is not looked after, serviced correctly, or repaired correctly is most probably not safe or roadworthy. Any vehicle bought from a private individual or even a dealership that is not inspected means you really don't know if it is safe or roadworthy.
You could argue that if a vehicle is sold as a category C or D then you are being informed of its history and can make better informed choice as opposed to the thousands of vehicles that have a hidden history because they have been repaired by the insurance companies.
Insurance companies keep the cost of premiums down by selling the cars they will not repair to the trade who can repair them more cost effectively. If they repaired a car then loaded the premium OR loaded the premium on cars repaired with a category C or D listing no one would take back their repaired car OR buy a cat car SO ALL insurance premiums would go through the roof. They would be cutting off their own foot.