I'm guessing you might be worrying unnecessarily.
If the engine didn't loose any fluid, i.e. got hot but then cooled naturally and didn't actually boil up I would think you don't have a lot to worry about. Regarding the fluid, if the collant bottle is clean with antifreeze then that should be ok and may not need a flush, obviously will need topping up with fresh antifreeze when the thermostat is replaced.
Your problem might be refilling the system, Antara radiators have a vent plug on the top right hand side corner when viewed from behind. its important this is removed when refilling to remove the trapped air in the radiator. You may also need to keep an eye on the coolant bottle for a couple of weeks as gradually any trapped air in the rest of the system may dissipate and the level will need topping up.
Replacing a thermostat is not the end of the world. A good tip if it does fail closed is to open the windows and put the heat/blower on full, probably on the screen, this helps remove the heat from the engine until it can be repaired. Richy