Please read on as to my experience re Cat C and D.
To my mind, the question would always be- is a small saving on initial cost worth the risk of using a car which has been written off by an insurance company? There's not just personal risk, physical and financial, but also the nagging thought in the back of your mind throughout ownership and the added grief when trying to sell.
I've bought a car previously, through an auction, which was cheap and was a Cat C, though I only found this out later- my fault, I should have been more careful. A Cat C can be a pretty serious issue, more so than just Cat D which is an "uneconomic repair". The repair to the front nearside suspension, of which I was unaware, began to fail on a long journey through rural France, causing even more damage including to the gearbox and sump. The subsequent repairer said the original work was sub-standard. The result- a £1500 bill- fortunately I had taken out after market warranty to cover the costs. I was lucky...only a little out of pocket, I got to my destination with a noisy but still functioning car, and I'm still alive.
There are loads of 500s around- they are not a rare car.So why buy a written off one- especially if you don't know why it was written off, and more so if you did not witness/ know nothing of any subsequent repair? At least have a full inspection done before you part with money.
My wife had a 18month old Pug 1007 which was struck by a 4x4 to the nearside.The insurance company gave me the option- £500 more than I'd paid for the car, or a full repair which would result in a Cat C car. I took the money and ran...........
I can understand the rebuilding of accident damaged cars if they are rare. Rowan Atkinson had his seriously damaged £850,000 McLaren rebuilt, and it was still saleable!! On a smaller scale, I myself owned a rare FIAT Strada Abarth 130TC, which was hit by another vehicle.It was written off as Cat D, as sourcing the bits,particularly panels and plastic trim, was an uneconomical repair for insurers- so I bought back the salvage, sourced the bits myself, had a repair done which I was able to view and document throughout, and then had the car inspected- all was well, but still the car will always carry the stigma to some as having been written off. I declared it when selling it, and fortunately I found a buyer, but many demurred as it had been Cat D, never mind Cat C.
I hope this gives you some food for thought......