After repeatedly removing rust on my car, and after looking under several other Mirage's, a summary of the worst rust problems:
The new car had its seams inside the cavities waxed, and repeated five years later, the result examined with an endoscope. No issues to worry about there.
Reasons for worry are:
1) The exhaust needs repainting at least yearly if you want to last.
Why? There are no aftermarket sources yet, and the OEM exhaust costs a small fortune, about as much as fuel for 10000km, or as much as a rebuilt engine of a small car of another make.
2) The front axle body needs to be de-rusted, undercoated, and at least two layers of paint on top.
3) The rear axle body needs to be de-rusted, undercoated and at least two layers of paint on top.
The rapid speed of rusting of the axle bodies suggests it can not last ten years if not repeatedly de-rusted and repainted. This needs to be done at least once yearly if you don't want to fall into the trap of too expensive parts for worthwhile repair. The exorbitant Mitsubishi price for both axles is about as much as the car is worth today, not counting installation!