Changing the waterpump isn't too bad, but you'll have to pull the airbox, radiator, and aux fan, just to get to it. Since my car didn't have the aux fan, I didn't have to remove that. Once you get those parts out of the way, it's a matter of loosening a few bolts and pulling it out. It'll take a bit of force, but should come out fairly easily.
You'll also need more coolant.. obviously. One gallon of BMW coolant and one gallon of distilled water, mixed together.
All said and done, it'll take the better half of an afternoon from start to finish.