I think the 190E 2.6 5-speed is just as rare as the 300E manuals. And the SL is rare too...really, any manual trans MB from that time period is RARE. The 16V is probably the most plentiful.
The SL has the M104 but the block is almost identical to the M103. Both blocks will accept the same transmissions.
Out of curiosity, how much do you expect to pay for a 5-speed tranny and necessary associated pieces to convert your car?