Hi, first of all welcome to the Belta's owners and fan club
Installing 15'' inch wheels will raise the initial height of the vehicle by an inch. They should not hit the speed breakers. Take your car diagonally over a speed breaker the under-carriage won't the hit speed breaker.
@zubair530 has installed spacers in his Belta and he has no complain till now whatsoever. I however would not like to mess around with the suspension, but hey that's me. They are rubber spacers which in most cases are installed on top of springs in order to raise the ground clearance of the vehicle.
No, 15'' inch wheels won't rub against the body if you are running the correct profile. If you're running lower profile tires the odds are that they will touch against the body on jumps. Get a proper profile, 195/60-65/R15 would be the best.
You won't be able to connect an iPod/iPhone into your stock built-in CD player. Yes there are many other options, you will have to get an OEM DVD/2-Din console in order to fit a DVD/MP3 player. They aren't easily available but if you do get your hands on one it'll most likely cost you in between 5-6,000rs.
I hope I have answered your questions, if you need any further assistance feel free to ask us.