They have committed no crime save possibly misfeasance in public office. That is not really runnable. They had the right to buy the cars. The only subtlety is that, when the cars were ordered, the cars had features, extras perhaps, which the officers liked. Now, where is the evidence that this was some dishonest conspiracy to ensure that nobody else but the officer concerned could buy the car and did so at an advantage to himself and a loss to the public purse? It may be that the officer, having regard to the discount, was offering more than the trade would pay for the car . You only need one bit of evidence that that was, or could have been, the case and you can wave goodbye to the prosecution of all of them. You'd never get close to satisfying a jury enough to convict.
As a disclipinary matter alone, it runs, because it could be seen as not in the spirit of the police force, it doesn't look good and could be seen as causing dissension in the ranks (and any other excuse !)