There are two kinds of contracts with property managers - one kind that only find tenants, and another that manages everything (e.g. if the boiler breaks they get a plumber).

Which kind are you talking about? I'd be surprised if the discrepancy between countries was as large as 6% to 15% for the same contract.

The 6-8% is for the ongoing maintenance and routine inspections. Usually leasing the property is 1 week's rent, however sometimes that is included in the management fee as well.