Thank you for your question. I'm always excited when women are doing maintenance instead of repairs! Good for you. :)
The Transmission fluid you should be using in your 400e is Dextron - which you can purchase at most any auto parts store.
The Power Steering Fluid should be Mercedes Power Steering Fluid - which you can purchase from your Mercedes dealer by just contacting the parts department instead of the service department. To help you even further simply give them the following part # 000-989-88-03.
Thanks for your question!
〉 Answered on Feb 23rd, 2007 by Kim Walker, Marketing Director at Peak Automotive