M103, M102, OM601, OM602 all were offered in the US with a 5speed and all the flywheels will interchange, so the right flywheel in the US is not a tough find. Its the heavier 617 diesel flywheel that is hard to find here.

As far as I know the 190E manual transmission and flywheel will bolt up to your M103 300E. Pedals and shifter box should bolt up to you car.

You will need the master cylinder to slave cylinder line fit for the 124, which you can make yourself from brake line and a brake hose, or buy from the dealer for something like $60 last I checked.

You will need a W124 manual transmission cross member, which last I checked is still available at the dealer.

You will need a longer speedometer cable, available new for like $50 from Pelican or others.

You will need W124 length manual shift rods, which are also available last I checked. Or you can lengthen the 190E ones.

You will also need to lengthen the 190E front driveshaft half. As far as I know the 260E automatic driveshaft is going to have a bigger yoke on it than the manual transmission will have. So either you will need to put a bigger yoke on the manual transmission, or use the smaller yoke driveshaft from the 190E. Either way, you're having a driveshaft made probably.

Others who have done the 124 swap will chime in if there is a driveshaft solution.

Budget maybe $1,000 to do the job? It's not unreasonable, but there are some odds and ends you will have to pay up for if you want to do it right.

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