In order to replace glow plugs on a 126 125,with a 603 6cyl engine you will have to remove the intake manifold and the cross over pipe.
You will need the manifold gasket and the cross over gasket.
A new set of glow plugs.Bosch is a must don't go cheap or you will have to redo this project all over again.
Tools needed.Also you want to invest in a reamer for the glow plugs.this will remove any excess carbon that has built up in the chamber.If this is not done your glow plugs will wear out prematurely and engine will be hard to start.
Cross over pipe removal.
Now you will remove the intake manifold studs with your metric allen key
Remove the old gasket and make sure to clean the cyl head and intake before re/assembly
Glow plug wires are removed
reamer is being used to clean the inside of all carbon.
Now time to put everything back together.
All these pictures refer to replacing the glow plugs on my Mercedes Benz.