What is so wrong with the OEM unit? Are radiator fans a real problem? This is what I'd do:
1. Buy a new radiator, expansion tank, cap, and expansion tank plate - $270
2. Buy a thermostat and new front hoses - $115
3. Antifreeze - about $20
4. Water Pump - $65
So, here you've spent around $470 and replaced the majority of the coolant system. If the fan goes out, you can replace it by pulling it right out of the top. The fan costs $360. So, for $830, you've gone through about everything. Remember, the hoses, thermostat and water pump don't last forever either.
As for that other radiator, you'll spend $1700 and that's just for the radiator. Now you must use the BMW coolant and change it every two years. So, you need to include that in your cost ($200 over 10 years) and the time it takes to change the coolant. I can tell you that a Spal fan doesn't last forever either.
If you think about it, your current cooling system has lasted (and is still working) for 90K and five years. Not bad for the price of the OEM parts. Then, you might want to keep the car for another 10 years and that might sound great today but things change (buy a new BMW, your car could get totaled, financial situation might change). So, do you spend $1660 over the next 10 years for a complete cooling system overhaul, or $1900 for only the radiator and fan? Then that radiator will loose efficiency as they get dirty just like every other radiator.
If you really are looking to spend some money, put an oil cooler on it. There's a reason why BMW doesn't put an oil temp gauge on their cars. Everyone would freak out.