Like many modern cars, rust is rarely an issue even on the early MINI's.......unlike the classic Mini where it is/was a constant battle to keep at bay!
On some early MINI's the only rust that sometimes seems to appear is hidden under the rubber door seal along the bottom edge of the door opening/sill flange. If you gently pull off the rubber seal and it is wet and rusty underneath, clean it up and apply some rust treatment and re-paint with a touch up brush. This is rarely a serious problem but if found best to treat it before it gets too bad, some early MINI's were rectified for this under warranty and the factory paint process in this area improved for future models.