Based on your usage, the 13" base model MacBook Air seems perfect for you.
Now is a very good time to buy one, there probably won't be a newer model before next summer, and as usual with Apple, price of the current model won't drop until then.
If you run out of the 128GB of storage, using a USB 3.0 external hard drive for your music and movies is a good idea since those don't require the high read speed of flash memory. To have a fast computer, what matters is that you keep your OS and applications on your internal flash memory.
Enjoy your new Mac! Especially since this is your first one I'm sure you'll love it.