Cat D is something like the damage is 60% of the value
Cat C though is a total write off, normally extensive damage totally more than the value of the car so certainly stay away from cat C
To be honest buying a CATD even if the damage was not immense can still cost you, insurance can be more and trying to sell on is a nightmare
My father recently bought and repaired a CAT C Focus ST, it just needed a windscreen and some windows replacing. The glass had also marked the dash slightly so I guess they must have put down the cost of the dash as part of the repair. My sister has the car now, didn't bother doing anything with the dash it's only a scuff mark.
He also bought and repaired a CAT D Focus RS, had the side smashed in and was a massive repair.
I wouldn't write off a car just because it's CAT C/D, as long as it's repaired to high standard and the price reflects its a write off then I'd have no problem buying it.
(My father did pay over the odds on the RS at £6k as it was more work then he originally thought - bought from online auction so was a bit of a gamble)
The ST has 50k on the clock though and was only £1300 so a bargain IMO
