It seems pretty clear from these symptoms that the engine is massively overheating. That explains the boiling water in the overflow tank which will be dirty mainly form rusty deposits within the engine’s cooling system. The question then becomes what’s causing the overheating.

There are lots of reasons for an engine to overheat, including low coolant level, a blocked radiator, collapsed radiator hose, failed head gasket, incorrect ignition timing, poor fuel mixture, a damaged or missing fan shroud, slipping fan belt, worn water pump and many, many more. But a good mechanic should be able to assess the situation, do a few quick tests and determine the root cause of the overheating, then you can move forward on a fix, knowing that you’re tackling the correct issue.

The other piece of advice is to not drive the car until it’s fixed. Continuing to drive with an overheating engine is a great way to destroy the engine completely. At that point you need a new engine or a new car.

The car is now 11 years old, so you would expect it to be worn to some degree and using oil as a result, so putting oil in once per month is not a big deal, providing it's not a lot of oil. I would keep a record of how much you are putting in and show it to your mechanic the next time you get it serviced.

IF IT'S been regularly serviced, the chances of having a problem in the next 3000km are pretty low, but I cannot guarantee you will not have a problem and I don't like recommending that you miss a service. I would have it done if it was my car. It will also make it much easier to sell. I would think you could ask $6000-$6500.