You can change the settings on the iPad so that if you buy a song or app…
iPad Basics Part 8 – Download Previous App Purchases
Once you have purchased apps from your iTunes Store, may automatically download to your iPad, iPhone, or iPod. But for some reason you have deleted your app you can always download it again.
This movie shows how easy it is for you to download your apps once they have been purchased. Your iTunes account remembers all the apps that you purchased and allows you to go back and download any that are not currently on your device.