Replacing the water pump on a M103 is a straight forward, logical procedure. If you have never done it before it can take you up to 8+hours.
The Wiki is a bit brief.
You need to:
(a) Remove upper radiator hose.
(b) Undo fan shroud and move it out of the away
(c) Remove lower radiator hose.
(d) Remove fan and fan Clutch
(e) Loosen the bolts on the fan clutch, water pump and steering pump pulleys. Do this while the Belt is tensioned it will help to get a grip on the bolts on the pulleys if they are stubborn.
(f) Loosen the tensioner at the 19mm bolt
(g) Undo the tensioner adjustment 13mm bolt
(h) Remove serpentine belt.
(j) remove fan clutch pulley,
(k) remove steering pump pulley,
(m) remove water pump pulley,
(n) remove the Y bracket
(p) Remove the Belt tensioner shocker
(q) Remove the belt tensioner.
(r) Remove tube to water pump that feeds the heater.
(s) Remove the water pump.

The steering pump will be moved out of the way to enable access to the water pump.
Get a new pump and reinstall everything in order.
Replace the small radiator hose between the pump and head.
There is a small O ring between the pump and engine block.

Allow - just in case - for a new serpentine belt and belt tensioner.