I would say rust is a big fault with MX5s. I am a big fan of the car and have 2 of them. Rusty sills, rusty chassis, and rusty suspension are the main culprits. Most are easy to fix is you know a friendly welder or can weld yourself. My advice when buying a car, look on line on the DVLA website at the MOT history of the car and you will see if it has ever failed an MOT or has any advisories for rust. It will point at what to look at to see if it was fixed properly. If the registration is hidden or they won't divulge it to you, beware.