I just replaced the water pump on my 89 300e last week. If you have a good mechincal sense about you then I imagine you can do it. I am nothing more than a shade tree mechanic myself.
It takes some getting to thugh. You'll have to remove the radiator and fan. You'll have to remove the belt tennisioner befor you can get to the water pump. It's only 4 bolts but there kinda hard t get to . You'll definetly need extensions and swivels. I would also replace the small hose between the water pump and the thermostat. It usually starts to bulge out.
It's an all day job if it's your first one.
I found a new waterpump for 150. dollars through a salvage yard. I think german star has them for around 140 plus shiping
good luck