Has it been repaired or are you looking to buy it to repair?

Cat D means it can be repaired and put back on the road without inspection.

Cat C will need to have a VIC check done by VOSA before it will be allowed back on the road.

I think the category of the write-off stays against the car but I have heard of some challenging the decision to class it as a D or even getting the category changed from C to D etc.

Hope that helps a bit.