Replace EVERYTHING if your going to do the cooling system even though your only at 75k miles. One thing about BMW is that the cooling system and belts/tensioners are the first things to go.
I learn from the beginning since owning my 02 330i for more then 10 years is to use OEM and to replace things as "preventive maintenance". The Stewart water pump is a waste of money. I'm sure there is a ton of us, including myself who used the OEM waterpump with zero problems. You can get the entire cooling system overhaul as a package for a great price at getbmwparts.com
The only time the car has let me down was years ago, alternator died a few thousand miles out of warranty. Other then that, 10+ years of problem free driving.