On the whole Cat C/D thing, you have to use your common sense. I wouldn't touch a newish car that was CAT C or D. If the thing is worth (say) 12K and it is uneconomical to repair, it has been in a big smash. However well the repair was done, it wouldn't be anywhere near factory.
Now a 10 year old car worth £2K - a CAT C is a perfectly reasonable proposition.
My mother's Vectra was CAT C'd 3 years ago when someone rear ended her. The car was worth about £700, repairs (bumper cracked + tailgate dents) would have cost about £1500 with new parts, so CAT C. I got a new bumper (in the right colour) off eBay for £40, and screwed it on, and we decided that the tailgate dents were "patina", so we lived with them.