On the rear diff, open the fill hole before removing the drain plug. I used Redline Gear oil for my C-230 diff. I used a siphon pump from Griot's garage to fill the diff, and for around fourteen dollars I purchased the specific Hazet tool for removing and tightning the rear diff plugs (fill and drain plugs)
The transmission may not be a DIY job, you need a lift, extreme cleanliness, the Fluid is expensive and can only be the Mercedes Specific fluid. Ideally someone who had done the fluid change before on one of these trans should be the only one to do the work on your car. You need also to purchase a drain fill dip stick, and know the techniue for removing and replacing the drain fill (security) plug.