Regardless of what happens with you and the seller on whos paying etc, with something like that you'd be better to spend the extra cash rebuilding your own imo. Then you know what you have and it wont be lying down the road on you in the middle of the night. It wouldn't be a quick in/out change anyhow and you will be paying someone by the sounds of it, to do the work. If you buy a second hand box, you don't know the history and it could just be as bad and then you end up building it anyhow along with the cost of 2 box changes in and out and be worse of ones wise. Either way its going to be a costly one, but that's to be expected when things like that go wrong on cars of that spec. They were very expensive new, and the price of parts unfortunately don't change with the age of the car.