The W124 chassis is (I think, correct me if I'm wrong) the most popular that MB ever made, and for good reason...it is a strong, long-lasting vehicle that doesn't require a lot beyond average maintinence...assuming that it has been done religiously. My personal experience is with an '86 and it now has 204k miles on the clock and still runs like a champion. I thankfully can't report first-hand on accident safety, but from what I have accumulated on that subject the model is excellent in that respect! As far as problem areas, the only one I can comment on is the heating/AC blower motor and the electrical fuse that connects it. It is a problem that WILL surface if it hasn't already, but a replacement of the blower and motor ($150 parts + labor) as well as removal of the fuse and addition of a circuit breaker ($100 p & l) will correct it ($500 total +-). Not bad for a car pushing 19 years old. Again from personal experience...the transmission leaks fluid because it needs a rebuild of seals, but still runs like brand new...seriously.
If I had to give an opinion regarding a 300E for a teen (or anyone for that matter), I would say most definitely!!!

__________________
1996 SL320 97K miles
1996 C220 130k miles
1992 500SEL 170k miles <---sold
1986 300E 216k miles <---sold