The situation with John has a number of considerations. When he is considering his total weights is he using the manufacturer's kerb weight, which is very often lighter than the actual weight of the tow vehicle. It could be that he is either on 100%, or maybe just under, if he took everything to a weighbridge, so is not illegal. However, he is certainly looking at towing more than the maximum towing limit for his car and although the salesman maybe should have known that, it may be he just used the weights. From his description the salesman stated it was a 'good match' not that it fitted the 85% guidelines, or any other limitation. Now if 'John' quoted his maximum towing weight to the salesman, and then received 'in writing' a statement that it would be fine, then he has a legal case. Verbals, I am not so sure. I suspect a Court would ask why the customer had not made more enquiries.
As far as his deposit is concerned, again as far as I am aware, there is a 14 day 'cooling off' period for Contracts so he can cancel within that time and get his money back no problem. I would also suspect that if it is a reputable dealer, then they will be flexible, especially if perhaps 'John' was to offer to buy a lighter van from them.
Again, I come back to my main plank of contention, that it is up to us as customers to ensure that what we are intending to purchase is a) exactly what we want and b) is what we REALLY want and c)it is always best to think things over before signing. The good deal will always be there tomorrow, if it is such a good deal.