Considering your budget and the desire for fuel economy and easy maintenance, it's great that you've already done some research. Among the options you've mentioned, here are some insights:
Cultus AGS (16-17 kmpl): The Cultus is known for its decent fuel efficiency and affordability. It's a popular choice in its segment and can be a good option for your requirements.
Nissan Dayz Highway Star S Hybrid X Pro Pilot: The reported fuel efficiency can vary depending on driving conditions and fuel used. It's worth noting that hybrid vehicles like the Dayz can offer better fuel economy in city driving due to the electric motor assisting the engine. High octane fuel might improve efficiency in some cases, but it's recommended to consult the manufacturer's recommendations and consider the availability and cost of high octane fuel in your area.
Mitsubishi EK X (18-20 kmpl): The EK X is another option with reasonable fuel efficiency. Mitsubishi cars are known for their reliability, so it could be a good choice for easy maintenance.
As an alternative, you might consider looking into models like the Toyota Passo, Honda N-One, or Suzuki Wagon R. These vehicles are known for their fuel efficiency and could fit your budget and requirements.