This is what I recommend.
Buy a pair of AU Falcon thermos with integral shroud from the wreckers. These fans really kick ass and are a firm favourite with Aussie muscle car tuners; they move 3,000cfm air measured with the radiator in place (unlike some aftermarket fan vendors - eg DC - who quote figures for freestanding fans).
Should cost around $70 to $120. Don't be tempted to buy the cheap Chinese copies off ebay - the OE Ford items have the strong Bosch motors, the ebay ones don't.
I've got them on my V8 Rovers and they work much better than the old engine driven viscous fans. I think Larry Perkins once said they were worth an extra 12hp on a V8 at 6,000rpm over the mechanical fans.
Now cut them in half (they're all plastic) and fit one fan with its shroud to the Mini radiator. Remove the engine fan and front shroud and bolt the radiator with brackets to the inner wing - makes removal a five minute job.
Then buy a variable speed Delta Current Controller from the US.
The DCController uses a temperature sensor inserted between the fins in the radiator near the radiator outlet. The controller controls the fan speed by use of PWM (pulse width modulation) which is an efficient method of slowly ramping the fan speed up and down, which avoids large current surges that a simple relay control allows.
Brian Baskin makes these in his spare time in Navada. He doesn't deliver real fast usually (I waited 3 months for delivery), but his products are top quality and have found their way onto countless Mustangs and other US heavy metal that require serious cooling. Just check the Mustang forums to see how highly regarded this item is.
http://www.dccontrol.com/introf0.htm
Cost for the FK50P Controller with all wiring and attachments included in the kit is around $200 delivered.