This is a 3.4 engine, so thermostat replacement is not one of the easiest to do, however, the price should be under $200....easiest way to do it is to pull the throttle body....you will need a gasket for the throttle body....
We are also talking about three coolant components, so draining and refilling is figured one time......
How much was the coolant leaking BEFORE THE SEALER WAS ADDED?
Sad part is you can replace all three and still have a problem.....
FIRST THING DONE, should have been to find out where the leak was coming from....was it the w/p? If it was, than that is one of the easiest w/p's in the world to replace, a DIY'er can easily do it....
If the car was not overheating before and not leaking much coolant, that sealer could have compounded your problem......
These engines are known for lower intake leaks and should have been one of the first places to check....
We have no idea if the sealer actually clogged the radiator, or if this was just a progression of the original problem.....for example, losing coolant creates an air pocket......just adding coolant does not get rid of the air pocket.....finally, the air pocket gets to a point where it blocks the flow of coolant through the engine and it overheats......this is usually validated by lack of heat from the heater core, because it is air bound....