I replaced the water pump on the 300E (103 engine) at about 120,000 miles. I went with a new MB pump as well (about $100). My belt tensioner was showing signs of wear too, so I swapped it while it was apart (about $120 I think). I also replaced the short hoses, the thermostat and the serpentine belt. It's been a while ago, but I think I spent about 4 hours on it.
Experience is something you don't get until just after you need it.