Why would a cat c write off make a substantial increase to the insurance premium? The vehicle should be road worthy post repair, as it would have needed to have been re MOT'd, the only difference I forsee on the insurance is the vehicles value, as if it has been written off before, they will value it less than an equivalent vehicle which hasn't been written off in its past.
Poor stability at speed? Something not quite right there, without looking at it it's difficult to know whether that's down to poor modifications or not. The spray liner looks nice around the bottom half, something I've been interested in doing but I've never had an 80 that needed tidying up around the bottom half.
With the write off, stability issues and rear locker U/S there's plenty of opportunity for negotiation.