the tranny filter for your E320 is inside the transmission oil pan and is very easy to replace. The power steering filter is located inside your power steering pump resevoir. Just unscrew the top cover, suck some of the fluid out (or drain it) and then remove the stem that the cover attaches to and that's when you'll see the filter laying in there. Takes only a few minutes to replace.
Back to the tranny filter though, drain the fluid from the drain plug located on the pan, then remove the bolts holding the pan on and you'll see the filter. Don't forget to drain your Torque Converter while your at it. This is a very easy do it yourself job and well worth the hour or so it will take you to do it. Whatever fluid you use is mainly up to you, I believe Dexron II is the only spec although Mercedes does have a list of approved fluids for this tranny. A lot of people here use Mobil 1 synthetic ATF, Amsoil synthetic ATF or Redline. May see some benefits of synthetic if you live in a very cold climate, even if you don't you still might like it, I know I wouldn't use anything but synthetic oil and ATF but that's just me!!