If you read this board and others you can find many E60 owners well in the 100,000 miles and have not replaced the cooling system.
I have noticed a lot of people on these boards tend to post all the negatives and none of the positives.
The E60 is my 4th BMW and I've yet to replace any cooling system on any of them. My E32 went to 186,000 miles before I sold it and only did the oil pan gasket.
As long as you don't overheat the car it will be fine if something fails. Might leave you stranded though. So it is a gamble. Dump tons of money into it and drive without worries or push it and see what happens.
Good luck either way.