The coolant renewal is pretty straight forward.
Make sure the air is not on recirculate. Undo the coolant expansion tank cap, make sure the engine is not hot otherwise hot steam will spray out of the expansion tank. Open all the bleed screws some on the radiator itself, some on the pipes radiator pipes going to the bulk head. On a 1.8 you'll have an undertray, i.d drop the front down so i'd get access to the lower pipe on the radiator then i'd undo the pipe carefully and then get a bucket and pull the hose off completely and let the coolant run into the bucket.
You could flush it out or not your choice, then to refill, reattach the lower hose securely. Refit engine splash guard. Then starting at the lowest point bleed screws start to refill the coolant, squeeze the lower and upper radiator hoses to help expel trapped air, once coolant starts to bubble out, close the bleed screw. Then Do the next for the same and so. Until all bleed screws are securely tightened.
Continue to Fill to MAX. Then refit the expansion cap loosely. Start the engine and run it at idle. Since your car has air con get a friend to help and get them to raise the revs to 3000rpms every 30secs while the engine is warming up. When this is done carefull open the bleed screw on the heater hose, bearing in mind the dangers of hot coolant and bleed out any trapped air.
Add more coolant so its at MAX, but only for the first few mins after the engine has started otherwise the coolant will get hot.
Once the coolant level has stabilised you can now tighten the expansion tank cap securely.
Let the engine warm up until normal operating temp, let it run for a few mins then switch it off and allow to cool.
Check for leaks around the hoses and items you've messed with and check the coolant level, top up if necessary. Remember to check coolant only when its cold for an accurate measure.
Regarding the gasket/o-ring it'll come with the new thermostat.
So Good, Fiat Want It Back!
