Cat C means it has had serious damage (and written off).

The difference in cost between a perfectly good and honest s/hand small car and a Cat C one could be peanuts if you buy well, so IMO....not worth the bother.

Cat Ds can be ok, depending upon model, as insurance will often write them off even though only minor non-structural damage has been done (locks, cosmetics, etc) if the car is a few years old.

On the other hand I bought my boy's lovely condition 2 owner low-mileage 2002 Punto for 500 quid....straight car, so hardly worth the bother of risking a previously written-off one.