I hate to say this, but the best interpretation I can put on this is that he doesn't understand what's gone wrong and has installed a quick-and-dirty fix.
The worst case is he's had it overheat, caused some damage, bodged a fix and is shifting it fast.
You have to assume that there has been at least one overheating episode, when he first discovered the
problem. So the engine is compromised, it may be about to fail. I'd budget for a top-end rebuild and add that to what he's asking for the car. If you still think it's worth it, fine. I'd be asking for a price reduction because of the known fault.
I'd also want him to give a written history of this fault, when it was first discovered and how, how many miles it's done since and how many overheating episodes it's had. Does the coolant need constant topping up and is there a mayonnaise-like deposit inside the oil filler or oil in the coolant header tank? All of those are pointers to a head gasket failure, the first likely casualty of overheating.
If it's a quick and easy fix then why hasn't he done it himself already? The thermoswitch on the radiator is an easy swap-out, and not expensive. The fan control module is less easy, and the fans come in at over £100, I think.