You could also get the AA to test it. I think it's about R500, but they will/should check everything - tell you if it's been in major accidents, structural damage, etc.
Mileage does sound very low. Would they reset the mileage if the engine has been replaced?
Maybe it's been clocked.

Although, aren't those the Opels that did "the moon run" back in the day? ie. drove the equivalent distance to the moon in one go (?)
EDIT: Although the distance to the moon doesn't come close to clocking it. Still impressive that they could do it.