It's the same basic engine as the other supercharged AMG cars. There are no real problem areas, but make sure the engine has not been modified. People like to hop these up through larger supercharger drive pulleys and custom ECU software. An additional 100HP is easily available, but then sometimes the motor does go boom! As well, the ECU tunes remove the torque limiting during gearchanges, and then the transmission does blow up.

There are more minor things to break on the engine. The supercharger clutch sometimes fails. Replacement is easy, but factory parts cost is $1K. The pulley bearings for the serpentine belts (two - one for accessories, one for supercharger) seem to wear out inside of 50K miles. A couple hundred bucks, depending on which one fails. The belts themselves tend to squeek. I replaced mine at 35K miles. I think the underhood heat did them in. The intercooler pump has been known to die an early death. Again, not too much money - but leaves you without boost until it's fixed.

Be aware the Kompressor drivetrain is somewhat less refined than a non-AMG car. The engine occasionally misses at idle until it's fully warmed up. There can be a quite sharp lurch when the compressor clutch engages. And fuel economy is abysmal - it bet it's tough to get 20MPG highway in an S55.

Be sure to check other cost areas of owning an AMG. In particular, the brake rotors on some AMG models run ~$900 each - a brake job can run $3K or more. Not sure about the S55, but this is true of the E63.

And of course, tires will be more expensive and shorter lived.

Unless mom is quite the enthusiast, I would guess she'd prefer a 'boring' S-class.