No doubt there is sound logic in Mango's stance of pre-emptive cooling replacement, but none of the BMW mechanics I know agree with that approach. One guy (Will) has been involved in racing, repairing and rebuilding BMWs since the early 70's, and my best friend Alex is ASE certified and has been working on euro cars since the early 80's. Those two guys say you replace things as you need to, because they've seen factory E46 cooling system parts go to 200k miles and still be fine. But that also entails being a responsible car owner who takes care of their car and is alert for signs of problems.
Mango, I believe your advice is excellent for most of us who tend to ignore problems. I have replaced a leaking ET and admit to recently doing a pre-emptive thermotat replacement while I did a tensioner pulley because thermostats have crapped out on every euro car I've owned over 100k miles. My hoses are in good condition. The other key parts give off warning signs when they die and I will respond appropriately. I had over 200k on my E30 with the original radiator, lower hose, temp switch and fan clutch.