First off, there's no doubt that the Kenlowe fans are much better than anybody elses. But you do pay a very high price for the extra performance. I suggest having a look on Ebay where you will find a lot of choice at a very small fraction of the cost of the Kenlowe. I'd be inclined to see if I could squeeze a curved blade 16" one in on the grounds that that would cover a greater area of the radiator surface. It would be quite acceptable to cut a notch in the lip of the undertray to accomodate the depth of the motor. It will be covered by the grille so won't show. Didn't Testrider have a picture of that? The other key factor is the power consumption of the fan. Compare what Kenlowe quote you on power consumption with what you finish up buying.
As for the control system, there's just no way I would use either of the Kenlowe systems. They stick in my crawe as an engineer as being plain old lash ups! Especially when there are competent alternatives. The superb engineering of a Rover demands something better.
Luckily, as you guys have identified, there are alternatives. I'm quite comfortable with the in hose type mentioned above. But there is one further alternative, which is my favourite:http://www.revotec.com/catalogue_EFCM22.asp
These sensors screw into an existing rad fan control socket on the radiator. Most modern cars from the late 80's onwards are fitted with these. Ah, I hear you say, but the P6 doesn't have one of those! Well, not as standard, but any radiator shop will fit one for you when they recore or otherwise repair your rad.
So if you own a V8, then first step is to get a three row core fitted to your rad (the standard fix for dodgy cooling on the V8) and at the same time get one of these bosses fitted into the hot side tank. Then you don't have to cut a hole, stuff something under a hose or otherwise bodge anything.
There is also a fan controller somewhere which gives a proportional response to rising temperature, so that the fan starts off slow and gets faster as the car heats up. I can't directly find the link to that right now so I'll add that as I get to it.