Cat n im my opinion depends in a few things what the cat n was for exactly would be the first thing i repaired the front end of a zed 3 earlier this year for a forum member that was a cat n now it had damaged front so
Front of bonnet
Front panel
Full bumper cover with fog lights grilles etc
Bumper support bar.
Offside headlight.
That was just to get it back on riad still has some scuffs and dings.
This is Lynne's zed written off as cat d on the old scale for 2 paint tins full if screws and nails came off a collapsed garage shelf one going through the windscreen the other the bonnet.
Owner bought back got salvage got bmw to repair kept another 6 yrs ( spoke to previous owner) so different reasons for the same result .
This leads me to the second factor and that is how much you are paying for what you are buying and what it may have been through that Turquoise Green zed above cost me £800 with rear window damage and running fault.
Rest of it in very decent condition so once the faults fixed more than worth what i paid ( so needs to be decent value at at -40% or so on market prices)
Which leads to the last couple of things first is resale value less buyers harder to find.
The second is if you crash your previously written off car and it is going to be written off a second time most insurance companies will Not give you the car back it will get binned and you will get the payout as they dont want it on road with 2 write offs against it.
Stephen.