Another alternative?
I know your post sought electric fan advice. Since you are asking about a 2800 model, assuming it is stock, there is another consideration. Your fan clutch is probably worn (there is friction material that does wear in there.) Assuming your radiator is decent, if the current fan clutch is "lazy" or "non functioning" you have a couple of choices other than adding auxiliary cooling.
1. Replace the current fan clutch with a newer one.
2. Upgrade your mechanical fan to one of the newer viscous fan clutch setups found on later 6cylinder models. This does involve changing to a different style fan clutch. I believe you can search the archives for more detail on this subject.
By the way, many years ago I had an overheating problem that required pulling the head. Rather than repeat this problem and because I had a few electric fans available, I made the effort to install one. Turns out that with the upgrade to a newer 8, 9, 11? blade fan setup, the electric fan was overkill. In two years of driving it was never needed and I removed it as dead weight.
Good luck
For informational purposes here is what BluMax had to say on the subject:
"I have retained original E-9 radiator--with brass end tanks and have re-cored with triple core+installed upgraded viscous fan--the major cause of overheat with the E-9 & E-3's that I have observed through ownership of 5 early cars--they came with the original two row core. a wimpy five blade fan and the old type fan clutch that was the main contributor for failure--it would silently wear out unbeknownst to the owner and the first sign was an overheat usually in traffic where no airflow was available to kept its cool.
Also--when these cars were designed the German engineers designed cooling systems to operate in moderate European climates--not the temparature ranges experienced throughout the U.S.--they also assumed that owners would regularly service their cooling systems--a foolish assumption based on my years of active ownership and observations at my local Independent BMW shop--periodic service--by draining, flushing coolant and replacing with a 50/50 coolant/H2O solution+a new thermostat every 60K miles and all new hoses with fresh clamps as necessary every 100K will prevent overheat issues--but is almost universally ignored by even well intended owners.
Since thorough sorting of my cooling systems--never a problem here in sunny SoCal--have driven in 100+F temps. on Freeways sometimes bumper to bumper, desert running at high speed and mountain driving--without an over-heat.
consider this--the early brass radiator parts and overflow tank are by far better quality, they are repairable and more durable than new stuff--and suggest stay away from the plastic o'flow tank if possible--a real piece of junk!!"
