Is it not dependant on whether it's 12V M103 or 24V M104 engine?
No it is not. All M104 engines have 1st gear start (I think), and those started being put in the 300E in '93 (2.8 version) then in '94 the 3.2 version.
Apparently only some
of the '91's were produced with first gear start (looks like luck of the draw), and may have been determined by manufacturing plant (mine was produced in Sindelfingen sp?) but I doubt it -- and all '92's with the M103 came with first gear start.
Are you planning on purchasing a 300E that is in this 1st gear start "gray area" ?
'91 300E, 214K miles