For the availability, we would suggest you to please connect with the nearest authorized dealer in your city as it depends on their stock book. Follow the link and select your desired city for dealership details.
In general, the down payment remains in between 10-30% of the on-road price of the vehicle and ROI varies between 10-12% and it totally depends on the CIBIL/Credit score of an individual. So, we'd suggest you walk into the nearest dealership to know the final finance quotation and documentation as they will be the better people to assist you. You can click on the following link to see the details of the nearest dealership and selecting your city accordingly - Bike Showrooms
For the availability of the spare parts of Honda Activa i, we would suggest you walk into the nearest authorized service centre as they will be the better person to assist you. You can click on the following link to see the details of the nearest service centre and selecting your city accordingly - Service centre
Both Ativa i and Dio are the most demanding scooter. Honda Activa i churns out good mileage and power as compared with Honda Dio. So, in terms of mileage, performance, and stability Honda Activa i will be a good option in comparison to Honda Dio. Moreover, we'd suggest taking the test ride of the options so that you can understand the difference and finalize your decision - Bike Showrooms
For the availability and cost of spare parts, we would suggest you walk into the nearest service centre as they will be the better person to assist you. You can click on the following link to see the details of the nearest service centre and selecting your city accordingly - Service centre.