To summarize.
A U.S. Apple Watch has a North American cellular radio that only works with Carriers in Canada, U.S., Mexico and Puerto Rico. Since your iPhone shares its phone number with the Apple Watch, the carrier needs to provide this support, and no carrier is going to do that for a radio not specific to their region.
It is illegal for Apple to sell an ECG device in a country that has not provided government approval, And it is illegal for Apple to allow an ECG device to operate in a country that has not provided government approval. So when an Apple Watch is manufactured, Apple already knows what country it is being shipped to, so its software has information about whether ECG can be used in the country of sale. The Apple Watch has GPS, so it can know when it travels to a country that has not approved ECG. Israel has not approved ECG. When a country approves GPS, the next Apple Watch software update allows ECG use in the new approving country(s).
Apple Watch ECG countries
https://www.apple.com/watchos/feature-availability/#branded-ecg
As for Australia, yes you can get an Cellular Apple Watch that works in Israel, except Australia has also not approved ECG, so you would then be waiting Israel AND Australia governments to approve ECG.
And yes the Apple Watch Series 5 comes in 2 cellular radio models. North American, and the rest of the world. But if you want ECG when Israel approves the app, make sure you do not buy it from a country that has not given government approval, such as Australia. Dubai seems to get a lot users from India, as it seems to be a popular stop on the way home to India and the duty free shop apparently has very good deals, but no ECG government approval.
As to audiobooks, I do not know. Traditional media restrictions are often implemented based on IP addresses, but I'm mostly thinking about streaming services. Not sure what audiobooks use. Also do the restrictions apply to https://www.audible.com/ or just to audiobooks via Apple?