I have exactly the same car - well a wagon anyway.E200.
I cleaned the battery area in just the same way.
The other guys are correct, there IS a drain, it's the hole through which the drain tube from the windscreen gutter passes through.
I cleaned up the area under the battery with baking soda to neutralise any acid, worked a treat. Most of the electricals are raised up a tad, and they are mostly behind the black plastic cover that lives behind the battery. Obviously, don't flood the area as you don't want to soak the ECU or other stuff behind there.
Only problem I had was slight deafness for a minute or two as I forgot that the alarm would sound when I reconnected the battery, and my head was within 6 inches of the darn thing
I was able to disconnect the siren leads until I found the key fob and turn the alarm off.