What happens if i stop paying my car finance?

You need to talk to the finance company asap and discuss your options. Also contact Trading Standards as I believe to have a prima face case for a vehicle which was not 'of merchantable quality'. They will advise on this. It may also be that the Small Claims Court should be the route to recover your outlay from the dealer. Dealers are bound by 'The Sale of Goods Act' as opposed to private sellers who are not.