Any transmission fluid labeled DexronIII will be
acceptable. It is the successor to DexronII. MB
does occasionally publish lists of recommended products,
though any quality brand of ATF should be acceptable.
MB requires DOT4 rated brake fluid. Again, any quality
brand will work fine. A key point is that metal bottles
are far superior to plastic - they do a better job of
keeping moisture out of the fluid while it sits on the
shelf. As mentioned, MB recommends brake fluid be changed
annually. Doing so removes accumulated water from the
system (brake fluid is very hygroscopic - it absorbs
water directly from the atmosphere). Eventually, the
water rusts your brake system from the inside out,
causing expensive repairs.
Power steering fluid is a bit more difficult. It depends
on what is currently in your car. It *may* have ATF, in
which case you can top it up with your quart of fluid.
See if the PS fluid is nice and red - if so, it's ATF.
On the other hand, your car may be new enough to be
filled with MB power steering fluid. In this case, I
think the best alternative is to pick up a pint next time
you are in the vicinity of an MB dealer. I think this
fluid is relatively clear.