It seems very little work to have the car written off on such a new plate, i would get it checked out carefully!!! See if you can find out when it was written off and speak to the owner at the time.
It May also be worth getting an RAC vehicle inspection and as always make sure the Cat D is correct through an RAC check. As for value, by rule of thumb a cat write of is normally worth around 50% of a non cat vehicle, therefore this does seem quite high. Don't be fooled by nice alloys, you can buy a non cat D car for about the same, put a nice set of alloys on it for not much money and have no trouble insuring or selling it at a later date
1 in 3 Cars has a hidden history. Check it before you buy it.