I'm guessing you have the basic RCD210? Radio/CD player with a cubby hole underneath?
There are several upgrade radios that you could fit, but all would need and additional bluetooth module and Aux/Media In connections. As you don't have have the Highline DIS or steering wheel controls, I wouldn't go for an original radio as you won't get the full benefits of it. There are lots of aftermarket radios that offer all the functions that you want for a fraction of the price of VW stuff.
I'd also consider fitting rear speakers too, as the S came with front speakers only.