As a point of reference, my 1998 323i with 140K miles just now started having trouble with the cooling system. First it was the plastic thermostat housing, and then the water pump went out. I use a good independent mechanic, and neither repair was all that expensive. Knock-on-wood, everything looks good now.
Excellent!
No cooling system issues with the ///M thus far either. Only real problem was one of the CPS's and that turned out to be a 10 minute swap. It does still need a clutch however.
Unfortunately (or fortunately, depending on how you look at it), my purchase of a BMW has prompted other family members to car shop. Dad is still looking at getting a Mercedes, and now that the Explorer has died, my sister wants a Bimmer. She's highly motivated to buy, since she has no wheels at the moment, and this car is priced very well. She's been in the M5 a few times, and that pretty much sealed the deal as to what she wanted.