Probably a whole lot more than you wanted to know, eh?
I realize that ctbenz's axe must be fully sharpened by now, clearly he's in favor of a benz. Ultimately, however, this is your mom's decision to make.
You might consider having her read this thread. There are some good points made across the board in here. And while I've stressed some things both ways, I hope I've come across somewhat balanced, I certainly don't mean to lean too much either direction. There are strengths and weaknesses on both sides here.
And I have to say that all the negativity in here re: Japanese cars hasn't been anywhere close the experiences I've seen or those I know have seen. Too, for all the positive things asserted re: MBZ, there are always threads like this one: http://www.benzworld.org/forums/gene...ously-wtf.html
(and you can read some of buymybenz's other threads for even more, or you can just cruise the various forums in here, clearly MBZ has (ahem) some reliability issues, both new and used).
The bottom line -- and I'm sure you understand it -- is that your mom is doing the very best she can and making the best decisions she can both for you and for her -- and for her transportation. So offer her any information she wants, any you feel is appropriate, and then support her fully no matter what she decides
. Many, many years from now you won't remember what car she bought, but the relationship you develop as you move along into manhood will benefit you both.
Take care and enjoy the ride,