I would haggle on price.
Try them at £6500 and you keep the car which gives you a bit of wiggle room to perhaps settling on £6000 and you keep the car.
That is if you really want the car and there are no other faults like airbag deployment etc.
Simple enough repair to the quarter panel and stick on a second hand door, job done.
Expect the repair and paint to cost circa £2000.00 including vat and paint and all materials.
Cat N is a classification used by UK insurance companies to describe a car that has been written off due to non-structural damage:
Selling
It's legal to buy or sell a Cat N car, but the status must be clearly stated.
History check
The Driver and Vehicle Licensing Agency (DVLA) must be informed that a car is a Cat N write-off, but this information isn't in the car's logbook. A history check can confirm if a car is listed as Cat N.
Replacement of Cat D
Cat N replaced the Cat D write-off category in 2017. Cat D was used to identify vehicles with light or cosmetic damage.
Explanation
The "N" in Cat N stands for "non-structural". This means that the car's frame or chassis is not damaged, but other parts of the car may be, such as the brakes, electrical components, or body panels.
Repair
While the car can be repaired and driven again, it may not be driveable right away. Some safety-critical parts may need to be replaced.
Buying
Cat N cars can be cheaper than similar cars that haven't been damaged. However, they can be difficult to sell because there are fewer potential buyers.