As far as your fan goes, just look at it running and make sure it's spinning at a normal speed. If it is, and your voltage gauge reads normal, then the fan motor is probably fine. Those things move some air, so you should be able to put your hand behind it and feel a stiff breeze. If it doesn't feel like it's blowing very hard, or if the fan speed fluctuates or your voltage gauge fluctuates then you probably have a problem with the fan motor or power to the motor.
As far as the head gasket, if your head gasket is blown and causing your engine to overheat, it would either have to be blowing combustion gasses into your cooling system or sucking coolant into the cylinders. I know there is a test that the dealership can do to detect any combustion gasses in your cooling system. Short of that, you can do the following to diagnose a head gasket leak:
1. Check your coolant level and color. If you're losing coolant and don't have an external leak, it could be leaking past a bad head gasket. If your coolant looks like it has oil in it, you might be mixing fluids through a bad head gasket.
2. With the engine running but not up to full operating temp, open the radiator cap and look down in it for bubbles. If combustion gasses are getting past the head gasket into your cooling system it will often cause bubbles to rise up in your radiator.
3. Perform a compression test (or optimally a leakdown test) on all your cylinders. You'll need a compression gauge to do this... it's a worthwhile investment in my opinion if you want to maintain your own vehicles. A cylinder that has significantly lower compression than the rest can signify a bad head gasket, as can 2 adjacent cylinders with low compression (where the head gasket has blown out between them).
4. Check your oil consistency and color. If you have water/coolant in your oil it will make it look muddy or like chocolate milk. You might even be able to see on your dipstick if there is a film of oil over water.
If all these things check out, it's still possible you have a leaking head gasket but I wouldn't jump to that conclusion. And $1,400 sounds really steep to me to do a head gasket on one of these cars. I've never done it on a 2.3 engine, but having done it on 5.0's and a 351 I would say that's even too much to pay on one of those engines! From what I've heard it's pretty easy to do on the 2.3. I'd definitely get another opinion from a different mechanic.
In addition to testing for a bad head gasket, I would pull out that thermostat and test it in some water on the stove. It sounds like the car has sat for some extended periods and even though the stat is fairly new it could have accumulated some rust or deposits from sitting and now it's not opening all the way.
