I'll tell you...I've had a '94 Disco from new.
High coolant temps should be resolved by bringing the o.e. components back to spec...I can't imagine trying to / getting any benefit from a different fan clutch.
I've had mud between the radiator / air-con condenser - a thermostat that failed closed - hoses that leaked - hoses that collapsed.
I've never had an issue getting air out of the system on a drain/flush/refill...just make sure the heater temps are turned to "hot" and that you leave the radiator plug as well as the expansion tank cap off when you re-start the engine with the new coolant.
Yes, it will heat up / expand and dribble coolant on the ground...just don't let the dog lap it up and you won't have a problem...when air has burped out and before it gets silly hot...screw the rad plug in first and then cap the expansion tank.
Deal with the simple bits first --- make sure the rad is clean inside and out, replace the thermostat - oh, the gasket can be a PITA to line up since it sits vertically...hold it in place with a teeny dab of grease. - confirm timing is within spec...
And either get a thermostat with a teeny hole in the top edge or drill one yourself.
I'm sure there's a technical name for the little hole with the fiddly bit in it...but you know what I mean, right?
If not, find the old guy at the auto parts store...he'll know.
Best of luck...
K