I'm assembling a fleet of 123's for my three youngsters. I don't feel that the 300D's are slow off the line. the lower geering makes thim move out with some alacrity. My 126, on the other hand is s. l. o. w. off the line, but cruises close to 80 at less than 3500rpm as opposed to 65mph@3000rpm in the 123's.
I would also prefer that the kids learn in a manual trans. car, but they seem hard to come by these days.
...I ramble on. A diesel MB is the perfect first car. Reliable, Durable, Safe, Economical and the satisfaction of driving the best.