my car is a 05 545i with 81k miles, so what's the life expectancy of the water pump? Should I just go ahead and change it now, rather than get stuck on the side of the road in the near future?

yes

BMW engines have little tolerance for overheating. Pony up now or live with the sword of Damocles hanging over your head.
Replace with quality unit that has metal impeller, and do belts, hoses, expansion tank, thermostat, sensors and coolant flush also.
Good knuckle-busting, money saving Saturday for DIY - or pay the Indie.

30-130k :whistle: Seriously I've read of them failing everywhere and anywhere in that range so its pretty much a crap shoot. I think the recommended is 100k but just go for it if you want some peace of mind. Still going strong here at 87k on the N52

i can hear my impeller chattering at 55,ooo ......i guess i need to do mine as well....i would like to do it my self... any special tools required?

Thanks for the replies guys, also, what about the alternator? Should that be changed too?

I wouldn't jump on that one. Losing it could leave you stranded, but can't destroy your engine.
I bought a little cigarette lighter voltmeter which I use to monitor voltage and keep an eye on the system.

what's the voltage reading suppose to be? 13.2v - 13.8v

Mine usually ranges between 13.6V to 14.2V

My water pump lasted around 65k.