Yes, you want a temp Switch for an e-fan, not a sender/sensor(variable) as used for temp gauge
Like this: 185° Degree Electric Thermostat Temperature Switch for Radiator Cooling Fan New | eBay
And then something like this for upper rad hose: Radiator Hose Adapter | eBay
These DO need to be grounded, they will have a screw terminal for ground wire.
The temp switch is open until it reaches the pre-set temp, it then closes and Grounds to the bung(engine), this would Close the Fan Relay and fan would come on.
12volts-------Fan Relay(coil)----------temp switch----------Ground
The 12volts can be 24/7 power or Key On power.
24/7 power allows the fan to keep cooling after you shut off engine, it will then go off when temp drops below temp setting, some like that some don't.
Key On power is fine, that's is what you have now, when you shut off engine mechanical fan shuts off.
Having a manual OFF switch can be handy if you run into high water, keeps engine bay drier since there is no fan spraying the water around.