If you are non-resident you must declare any income you have earned in Greece. This applies even if you receive this income by renting out to people from your home country and the money never touches Greece.
You will also normally have to declare this income in the country where you are tax resident but can normally offset the tax paid in Greece through Double Taxation Treaties.
In any case, even if you do not earn any income in Greece, you should file annual tax returns as long as you own real estate property in Greece.
The taxes that you pay when you buy a property in Greece will normally depend on whether you are tax resident there or not.
Tax residence is a determined by a number of factors;
How long you spend in that country? Is it 183 days or more a year (not necessarily continuously). If so you are likely to be tax resident there.
Is your main home there? If it is then you are likely to be tax resident there.
Is your immediate family (spouse and dependent children) based there? If so you are likely to be tax resident there.
Is your main economic interest there? If so you are likely to be tax resident there.
If you do become tax resident in a country then you will normally stop paying taxes in your home country and start to pay taxes in the new country.
Do not be tempted to have selective amnesia when it comes to declaring taxes