Hello there,
Sorry for all your problems.
Unfortunately, everything that you were told could very well be tied together.
Usually when a car shop replaces the head gasket, they also replace the thermostat.
What possibly happened is that your thermostat failed resulting in your car overheating which resulted in your head gasket problem.
Replacing the thermostat is the least of your concern though because it is a very simple and cheap job.
Symptoms of a head gasket would include:
-white smoke in exhaust
-coolant in oil
-hard/long cranking to start the car
-oil in coolant
-low compression in one/several cylinder (hence the hard cranking)
-loss of power
Have they tested your coolant for exhaust gas in it?
That would also indicate a bad head gasket.
$1500 sound about right for this job.
I would make sure to have it done by a mechanic you trust though; when you replace the head gasket, you should also have the head resurfaced; if done improperly you'll be back at the shop within weeks with another head gasket problem.
Also, a blown head gasket is usually the result of another problem. In your case it seems that it would be a bad thermostat; but make sure the technicians inspect your cooling system for any other possible problem.