Replacing a water pump on a M103 is a long process - if you haven't done it before. If you have any mechanical bent it can be done without special tools other than long thin extensions for your sockets to undo the water pump bolts.

You need to remove a fair bit of individual components to get at the pump. Allow 6 -8 hours to do the job by yourself for the first time.
The job is fairly logical and methodical. You need a new pump, all radiator hoses plus new clamps, thermostat and sepentine belt. (don't screw with old radiator hoses and clamps).
Get new coolant - correct stuff for 103 engines.

What I remove to do the job.
Fan and clutch.
Fan, water pump and power steering pulleys.
Sepentine belt, tensioner and belt adjuster.
Undo bolts for power steering pump assembly and move aside.
Heater tube running across front of engine into water pump. You get a new O ring for this in the water pump kit).
I remove the oil dipstick tube and first and second injector to get at the top right bolt of the water pump (Others don't do this - depends on how much room you have)
I also remove the distributor for easier access.
here is a image of everything removed.