Another useful phrase is "Defective goods". It sounds similar to not fit for purpose but has a slightly different legal meaning. It might get their lawyers jumping a bit because if you prove that the vehicle was sold in a defective state then you are on for a full refund and compensation plus there is the danger to life and limb element.
Good luck with it.
BTW that metal doesn't look 1mm thick even - did you measure it?
"Never have a stupid argument with an idiot - he gets a lot more practice than you"
there I go again
not too hard really