I have 58,000 on my '98 C43 AMG and no problems -- I suspect that the AMG single engineer hand assembly and (at least in the case of the C43) tuning refinements to the already proven and robust 4.3L V-8 are likely to give longevity better rather than worse than the already solid normal M-B engine. In any case, owner care and maintenance will be the most important determinant.
AMG wear items (e.g., brake pads and rotors) are expensive, but not an everyday affair.
'98 C43 AMG
'93 190E Limited Edition (including factory Sportline package)