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Originally Posted by awdirtdrifter

I don't have specifics on the 5-series cooling issues, but for the e36 3-series, they're as follows:

Radiator has plastic neck that is prone to breaking. Replace.
Water pump on earlier models had plastic impellers that were prone to breaking--not good. Must replace with composite-impeller pump.
Thermostat housing was plastic. Replace with aluminum version.
Fan clutch would go with high miles.
Auxiliary fan would sometimes go (not fun and expensive).
Random hose leaks (somewhat normal I guess).
Coolant level sensor would sometimes go (not really harmful but you would get the annoying "Check Coolant Level" message on the computer).

That's all I can think of at the moment. For my e46, the newer car, we had one catastrophic failure. It was the final stage unit, a fairly cheap electronic part that tells the fan when to come on. Stuck in a drive thru, fan didn't come on. By the time I noticed, temp was pretty high. Stopped the car and turned it off, KABOOM! Coolant everywhere and smoke under the hood. Engine was fine, only damage was the expansion tank. The elec unit that needed replacing is probably really cheap, but problem was, I had to buy a whole kit, which included fan, rad, shroud, condenser. That meant $$$!

Sorry for the ramble, just don't want you to get screwed. If he's leaking coolant, who knows if it hasn't overheated. If it has, it may be on its way to a leaky headgasket.

2nd on the cooling dept