the turbo never came with a maunual gearbox from the factory afaik
as a swede (northern europe)
about half of the w123s is automatic, but the smaler engines a lot came with manual transmissions.
the fivespeed fitted to the om617/m110 engines are extremely uncommon, (both as a ironbox in the w115/w114 and early w123s) and as a whole-alu 717.400 gearbox on the later.
the 200/200T/230E/230TE m102 (witch you US people never got afaik
) is the most common with fivespeed over here, the gearbox in that one is a 717.401 and made by getrag, but if you have a regular m102 with automatic or fourspeed, any fivespeed from a w124/w201 will fit directly to the engine, and just need some driveshaft and shifter fix.
Know of people over here that has done the fivespeed swap on their automatic or 4-speed m102 w123s.
the m102 doesnt have the starter on the same side of the engine as the rest of the w123 engines, and thus, isnt a compatible gearbox with the diesel/m110 engines etc, it is however compatible to the w124/w201 engines.
since manual gearbox availability is pretty scarce for the US-w123s, and the w124 engines have a lot, a pretty deacent option could be to fit a w124/w201 m102 or m103 engine in your w123, aswell as a fivespeed gearbox. thats what i would do. or the om602/om603 engines with the same type of fivespeed.
if going down the route of a fivespeed, do fetch one with a dual-mass flywheel setup, (different gearboxes for different flywheels)
it gives a lot smoother ride, swapped on my om602 turbo w123 from a single-mass setup to a dual-mass setup, and its a whole other ride where the noises, shakes etc dissapeared.
the gearboxes from w202 and w210 fits those w124 engines to, but lack the mechanical speedometer, and thus requires extra work if you want the speedometer to work.
Mercedes manual transmissions | Mercedes-Benz turbo
the 717.401 w123 gearbox is what i run behind the om606 turbo equiped w123..
the 717.430 the single mass i broke behind the om602 and the dual-mass setup 717.412 what i have know