Hi Peter -- I changed my e39 over to an all electric fan about ten years ago, using a SPAL fan that runs off a coolant temperature switch. No problems, even with my driving, the V8 and the oppressive heat and humidity of the DC swamp area.
In addition, I changed that tiny little fan BMW put into the front of our cars to go with the a/c to a 14" SPAL fan. Because this fan is on the bumper side of the radiator, it needs to be a "pusher" fan. You need a puller fan for normal cooling on the engine side of the radiator.
The supplemental pusher fan in my e9 that goes on whenever the a/c is on is also wired to an 85 degree Celcius switch that I installed into the bung in my radiator near the lower radiator hose. In theory, this switches on the supplemental fan in the event my engine gets to 85 degrees C and the a/c is not on. (I'm not sure I have ever actually needed this). If I remember correctly, you bought your radiator from Don at the same time I bought mine, and they both came from the same supplier. If I have this right, you could install a switch in that same location and use it to operate a puller fan as your main cooling fan. This would leave the supplemental fan to go on whenever your a/c is on, which is the normal operation of our cars. The circuit to run your main engine cooling fan like this is would be a very simple electric circuit, with a flying fuse behind the headlights like the circuit for the a/c fan and a relay that engages the fan whenever the switch in the radiator is engaged. You could either run switched power through the radiator switch, in which case the fan shuts off when the car shuts off, or run unswitched in which case the fan will continue to run even after you shut off the car until the coolant cools down. My SAAB used to do this and I thought it was rather neat that the car continued to cool itself down even after I shut it off. You might have to research whether 85 degress C is the right temperature; I know there is more than one BMW switch that fits in that location so there are a couple of different temperatures to choose from.