You made a generous offer of £200 towards the repair which under no circumstances were you obliged to. It was a private sale and the "Sale of Goods Act 1984" is still applicable and the goods must be "fit for purpose". It was however a very cheap van with high miles, there isn't a court around that would rule in his favour if it was as described. He inspected it, was happy and purchased it.

I would tell him that you were willing to make a contribution to the repair but following his attitude and seeking legal advice you are now withdrawing your offer.