I dont think CNG kits are approved for the EON as of now and it will take some time for RTO approval, due to which the dealer is telling you to wait for six months. It would be a good saving if you get CNG conversion done taking your daily usage to be 50km. But its better you wait for car specific kits to be made available before you get the car converted. If you use the car for short distances, and not regularly, then its better you stick to petrol.
There are many options available for a 2 DIN music system for Hyundai EON in the market. You can check with a local car accessory shop to know about the options in detail. Also, you can check out some options on our online shopping portal. Please find a link below to check out the music systems http://shop.cardekho.com/car-accessories/music-systems.html