Stick with MB ATF
From what I've read - stick with the MB ATF. I don't believe that MB approves a synthetic ATF for the 400E / E420 transmissions. You should be fine with MB ATF if you change it at regular intervals. I've read of some problems after people switch their tranny fluid to synthetic.
I just bought my 1995 E420 in June. I've replaced the ATF, differential lube, brake fluid and coolant with MB products, and run Mobil 1 15W50 in the engine. The mobil 1 is expensive, but I go 6000 miles between changes, offsetting some of the cost (synthetic allows longer drain intervals). PS fluid had been recently replaced by the PO after a steering box reseal.
Keeping the fluids fresh is a good way to get piece of mind for relatively little $. You should be fine with MB recommended products and change intervals for the other fluids. MB approves synthetic engine oils in the M119 engine.
Hope this helps,
1995 E420 72k