Replaced the belt today on my SL320 W129.
Posts on this site appeared to suggest that this will take 3 hours on an M103 engine. On my engine M104 it took about 20 mins including figuring out how to remove every thing.
The procedure I followed was:
Remove the radiator fan shroud, which is in two pieces main shroud and plastic ring around edge. Pull out plastic 'O' clip on top of shroud and move plastic ring to direction marked 'open'. Place this panel close to the engine so that the main shroud can be removed first.
Next pull off two metal clips holding main shroud, one top right and other left next to oil cooler. The shroud should clear the fan and pull up and out. The plastic 'O' shaped shroud should be taken out next.
There was no need to remove the fan. Make a note of how the belt is threaded around the various pulleys, or use the diagram in the hand book.
Rotate the tensioning pulley using a 16mm socket on an extension bar on the centre bolt by rotating this clockwise. This should release tension on belt and revmove belt.
Check for any play on all pulleys and any water pump leakage. The tensioning pulley was springing back and forth and did not need replacing.
Place the new belt starting at the crankshaft pulley and follow around until you get to the alternator. Again using the socket turn the nut on tensioning pulley clockwise and put belt on final pulley at the alternator and release load on tensioning pulley by moving it anti-clockwise and the belt should be tensioned.
Replacement of shroud is reversal of removal.
On starting the car the squeeking noise went away and steering was a lot smoother. Photo of car attached.