First of all congratulations a lot. Do start your ownership thread once you get delivery of your lovely Polo.
1. Music system. My advise is to go with VW because aftermarket shop normally don't have the required connectors and today or tomorrow it will be detected by VAG-COM and may raise warranty issues.Outside its cheaper but i preferred peace of mind.
Alternatively you can ask them whether they will fit it if you buy from outside, then you need to pay some 600-700 and there will be no warranty issues.
2. Mud flaps are good with VW.
3. Solar film you can get outside, but i have my own reservations about it such as night vision and vision in rainy season so i did not get it. Be careful when installing on windshield, water should not spoil ECU.
4.Seat covers you can get outside i got it for 2500 only. I chose loose fit against tight fit as it requires poking hooks in original seats which get rusted later and comes out if not fitted properly. Secondly i don't like any kind of messing up with original things.
For anything you are getting outside, be there yourself and watch closely when they work on your car.
And congrates once again. You have booked the hottest hatch available in India.
Do feel free to discuss anything if you need.