In '96 when I bought a new E300D, I noticed in the Star magazine that they all came with a block heater, but you had to ask your dealer to install the cord.
I expect that by purchasing the cord, you will be able to have a block heater easily. I believe that this cord is a standard cord, you may be able to get one from a source other than MB.
As I remember, on my '96 the heater was on the rear, passenger side of the block, but I'm not sure.
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in