I use to and still do to a lesser extent buy and sell used cars with my son and whether I’d buy a Cat N (D) vehicle would depend on whether there is a comprehensive history file documenting its damage and repair history.
Although a Cat N (old D) refers to non structual damage relating to the chasis or body, it still leaves things like non structual suspension, brakes, electrical, water damage, dame to the interior or body, so its bith vital and nice to know what happened and having a comprehensive history, documenting its damage and repair history gives you that vital resassurance of what actually happened for the insurance company to find it uneconomical to repair and how the repairs were carried out.
Buying from seller who’s not able (willing) to say or are unaware how the vehicle was damaged, leaves you wide open to buying a future lemon, because you will not know exactly what to look at and whether it was repaired properly!
The vehicle you are interested in should also have the service book which is upto date, V5C, 2 keys and a valid cotia MOT.
As @Oscarmax points out, if the write off has been recorded, any remaining manufacturers warranty will almost certainly have been voided and the insurance premium will be higher or you may find they won’t cover everything.
Cat N cars in many cases can be trouble free for years especially if accompanied with a comprehensive history of its damage and repair journey, without that you could be opening a can of worms. Another plauseable option if you thinking of going this route, is to buy what would be a Cat N, but oone that is unrecorded and have it repaired yourself, will still see you with a bargain Any inclin of water damage, walk away, it not worth it!