At least you know the pump has been changed.
I believe the thermostat fails in the open position, at least most ford ones do.
You can test the coolant temp sender by disconnecting it and checking the range of the connectors with a multitester. You should be able to find the acceptable range in a shop manual. The guage sender is more simple. you can disconnect it...run a test wire from the red/white wire on the connector to a ground, turn the ignition to on, if the sender goes right to hot...it is defective. There should definately be a wire attached to it, if it doesn't the engine won't function properly and will run rich because the computer will think it is running colder than it is.
