I would be alarmed about a 16 year old complaining that your car is too slow. When I was 16, I bought my first car, a seven year-old '71 Firebird Formula 400. I loved the car, but I'm lucky to be alive considering all the crazy stuff I did in that 350 horsepower car, with my crappy teenage driving skills. So rule number one is: ignore her complaints about speed. On the other hand, I personally think most MB diesels are so slow off the line as to be dangerous for a new driver when merging into traffic or crossing a busy intersection. An automatic 190 2.3 gas model would be a good choice, as would a 1980s 300E or 260E. The '91 BMW 318is is also a really sporty, great handling, economical, and not-too-fast car, but was only made with a 5-speed. If you consider one of these, make sure the profile gasket has been replaced. When it fails, coolant can spew everywhere, creating a dangerous condition. Other great cars for new drivers are Volvos. A nice 240 or 740 can be had for well under $6000, and they are safe, reliable, cheaper than an MB or Bimmer to care for, and will last forever. The non turbo ones are better than the turbos for reliability and longevity, and slower, too, which is a good thing for a new driver.
Good Luck
Colin