Whenever you change discs you always change the pads no matter what condition they are, always done as sets. The other thing with dynamic drive is that rear brake pads can wear nearly as quickly as fronts. As has already been said 3 buckled wheels can definitely lead to other issues with the discs, but that said most is down to previous ownership if they have consistently ran the brakes hot with aggressive driving, difficult to tell? That said I know pads and discs are very expensive from BMW, you could still use BMW discs/rotors and pads and get someone to install them save a fortune that way. I would not be disposed to upgrading the discs as BMW are one of the very few manufacturers that actually balance the discs in manufacture. Pads subjective as many like low brake dust etc., which BMW are not!!! A mute point you would be amazed how many ride their brakes when driving and don't even know they are doing it!!