Stick with the BMW coolant - it isn't that much more money. I did a complete flush to the orange Prestone Long Life stuff because I live a distance from a BMW dealership, and it has worked well for me but all things being equal I would have stuck with the BMW coolant.
Whatever you do, do not use the old style green stuff or any other coolant that contains phosphates.
And the water pump is dead simple to do - with a MY2000, I would definitely replace it. It takes a couple hours max.