Wrong. When the fan clutch goes out, the fan spins freely. There is a silicon based oil in the clutch that uses the viscosity of the oil and a bi-metalic spring to change the amount of oil in the fan clutch. Over time these leak / dry up, and now the fan spins freely and doesn't lock up and turn at the correct speed. The simple way to check this is when the motor is good and warm, shut down the motor and watch and see if the fan keeps turning after the motor has stopped. After it has stopped, grab the end of a blade and feel for any resistance...

On my Mercedes, when its fan clutch went out I was able to buy some Toyota fan clutch oil and carefully refill it. Its worked for 2 years since then with no problems.

The thermostat is an easy / cheap replacement and wouldn't do any harm, but they seldom fail, and when they do, they tend to fail open, not closed, taking longer for the motor to heat up.

I'd suspect that the radiator is fouled.

As far as the head or head gasket: Well, either way you have to take the head off. Once you get the head off you check it out. If it is a warped / cracked head, it really depends on where / how it is warped or cracked. Some good machine shops can repair the head.

There is one other possibility: If you don't find anything wrong with the head or head gasket, you could have a hairline crack in the block between a cylinder and a water passageway. This is a fairly rare occurance though, and typically if this happens, then what can often happen when the car is not running is that coolant will leak in through the crack into the cylinder in a large enough volume to cause the motor to hydrolock the next time you try and start it, causing REAL damage like bent con rods. This scenario usually doesn't happen with head cracks / head gasket leaks because their isn't enough water in the top of the head to drain down like this.

Other things to look for are oil in your coolant and vice versa. If you had oil leaking into the coolant while running, this can foul the radiator and cause overheating too. Oil in the coolant makes chocolate milk looking gunk. Also, typically, while running oil gets in the coolant more often, as the oil system is pressurized higher than the coolant system is.

If it was me, I would buy the factory service manual, identify any special tools I may need, read the procedures in the manual a couple of times to familiarize myself, have a clean workbench with plenty of plastic baggies and a sharpie felt pen, latex gloves and a digital camera at hand. I take lots of digital pictures as I go so I have a record of how it goes back together. Get the head torn off and then assess the damage. Also, since this is a V-6, if you are going to do one head, do both. Now is also a perfect time to do other maintenance items like the timing belt, tensioner pulley and water pump, have a radiator shop evaluate your radiator, possibly tank dip it to clean out any corrosion or fouling. (But chances are it would be just about as cheap to simply buy a new radiator). Raplace all belts and hoses and any other consumables like that.

good luck