cat D means it is either beyond economical repair from the point of view of the insurer or that the vehicle was stolen, insurance claim settled and recovered lets say with lock damage etc.
There is nothing fundamentally wrong with car from a safety point of view, so it is not going to crab down the road and fall apart. You are allowed to repair them and can be a great buy as you get them cheaper. However it is not a win win scenario. You paid less to buy it so expect less when you come to sell it, and of course you narrow your market as a lot of people have the wrong misconception about cat d cars, so would rather not buy, a bit like imports although this is now such a problem anymore.
you pays your money....
bernie400