Hi Rob,
I installed a Kenlowe Extra High Performance fan (16") a year ago which is the highest power rated fan they offer. I run a 4 litre engine and no engine driven fan and its gets hot under the bonnet.
I took the +ve feed from the spare post on the starter solenoid (live when ignition switched on). As Colin mentioned, I use a 40A relay and a 40A fuse as well.
Its worth considering installing a temp switch via a threaded boss introduced into the hot area of the radiator (top). This then provides the switch for the fan via the relay.
The Kenlowe I bought has a two speed option, but to date I have only needed the lower speed (which I presume has a lower start-up current draw).
Regards,
Simon