Where's the powersteering filter located? usually on the front right looking at the engine from the front of the car. Altohogh I am not sure for your specific model. Filter is in the pump at the bottom.
What's the differential fluid? and where's it located?
the differential is in the rear of the car if you have rear wheel drive the axles will run into it and the drive shaft join it.
I would recommend you by the manuals on cd they are available on ebay or mercedes directly.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)