Cat D is an insurance damaged category which basically means the car was not economical to repair, and it rather than paying for hire cars, etc. whilst repairs were ongoing, the insurer paid out market value. Someone then purchased the car in a damaged state, and repaired it and put it back on the road.

NOTE : there is absolutely NO evidence as to how well those repairs were done. The car is not independently inspected, and some repairers will really cut corners. So if you don't REALLY know how to inspect a car, think again. Don't assume an MOT means a car has been properly inspected, it's really a rather cursory examination.

It might be just as reliable, it might be terrible. See above.

A Cat D should be roughly 25% cheaper than a 'normal' car. Because most private buyers won't consider them, and lots of garages won't buy them off you in part-ex either.

Basically, if you are competent in inspecting cars, and happy to run it until it dies (you have to assume you'll NEVER be able to sell it at any price), then consider it.

If, on the other hand, it's your first car, and you don't know a highly trusted and competent mechanic to inspect it for you, and you'll probably want to change it before it dies, then steer well clear.