Great write up
A bit jealous of those US prices, if I had those prices here I could afford the overhaul every 30k miles
I had to do the cooling overhaul on every BMW I've owned, never replaced the radiator thou as I dont think it poses a danger at the mileage I had my cars with.
I havent heard of a rad failure before 100k miles so I personally dont recommend replacing it any time sooner then that. I was never for preventative maintenance and I still am against it. To me its just money wasted because I know exactly how long the parts can last before replacement. For example , early the water pump failures are usually on prefacelift automatic cars and are due to the fan clutch thats gets the water pump unbalanced. When I bought my current car at 96k miles , I took the water pump out and it slid as ROCK, no side to side play in the bearing whatsoever(pump was stamped '03). I still replaced it with a Stewart EMP because I was planning that anyway regardless of the condition of the old one, but it was quite interesting to see that OE one held so good. On the other side, I had an E39 530i before and only at 65k miles that I bought it with the waterpump was complete TOAST (way too much side to side movement). All E39s have a clutch fan (regardless auto or manual) so I can bet my money that it was the cause.
My current E46 is coming up for a rad replacement this summer, I purposely didnt replace the rad when I did everything else to see how long it would last, but now I think its time , dont feel like waiting anymore.
Another thing I'd like to add, I didnt see it mentioned or simply missed it.
The ET cap on our cars is the very big thing and gets overlooked WAY to many times. ET cap is a pressure valve in our cars, our cooling system is pressurized and runs at 2bar/29psi(on the bottom of the cap it says 200 kpa). Whenever the pressure gets higher the cap releases it (this is why you may see dried up stains of coolant on your ET eventhou its new and NOT leaking). Everyone knows that PRESSURE is the biggest problem , as its what gets our ET to crack. So, the less pressure you have in your cooling system the less chances you have of a failure.
E30s have a 1.4 bar/20psi ET cap (on the bottom it says 140kpa) which when installed on E46 will maintain a lower pressure in your system, thus having less chances of failure. Its a cheap and effective way to be protected.
'91 325i ET cap part # 17111742232
Another way to reduce pressure is to install a high volume water pump. We know that more water flow means less pressure .EMP Stewart is able to pump 20% more water thus reducing the pressure inside the system. They offer stainless steel High Flow water pumps that are also lifetime warrantied. This is the reason why I installed one in my car. Its not as cheap as the cap, this one will set you back $170 but in my opinion is well worth the investment