Is there any way to tell if/when the radiator has been replaced, and if the water pump has upgraded to the model with metal impellers? I'm looking at am e34 m50 that's on the edge of the service life for these things (109k) - maybe way over it for the water pump - and I'd like to verify whether or not I'll need to replace these things right off the bat.

Get the VIN number from the owner and call the local dealers. I did this and got print outs of any service done on the car I bought.(95 525I) If he got it done somewhere else he should be able to show reciepts.

I don't know of any way to tell externally on the WP, but the radiator should some sort of mfg info on top of it.

As a preventative measure, I would replace the WP, radiator, and entire front suspension (except shocks & springs) when you get the car.

I know that's not a cheap job, but the work is easy and if you're at all mechanically inclined you can do it all yourself. Tips at www.bmwE34.net (thanks to Bruno). I suggest the above because the cooling system can really screw you over if either one of those components goes. Also, the front suspension is really finnicky on the E34, when one component starts to go, it seems to domino across the whole front end. The car is at least 10 years old, so putting in fresh components will give you peace of mind for probably another 10 years.

If you want to do shocks/springs all around (which I highly recommend, btw), you'll be amazed at how well the car handles.

I've had both the wp and radiator go at rather inconvenient times, and I've been chasing suspension gremlins since I've owned my car. I wish I could go back and follow my own advice ;)