The pump assembly should last at least 70,000 miles and often quite a bit more depending on the condition of the coolant. I have replace the water pump on my 1982 240D twice in 279,000 miles (meaning the first, original pump and the second one I put in lasted about 140,000 miles each). The job is relatively straight forward. I did it in a parking lot once. You take the fan belt off as noted above, remove the fan, and the clutch assembly. Then the access in the area is sufficient to take the pump off without taking the radiator out. The pump should come with a gasket. I used a sealant with it just because the sealing surfaces looked a little worse for the wear.
So, I hope this helps, and good luck! Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)