When you purchase a downloadable item, you have unlimited access to your purchase and can download it as many times and as often as needed.
For videos, we recommend using VLC Player. It is a free download. Here is a link:
Apple does not allow videos to be save directly to an iPad or iPhone. You can use iTunes to sync a saved file from a computer to your device. We also recommend a third party application called Documents by Readdle. We do not own that application, nor do we support it; please download it from the iTunes App Store.
Here is a link to the Apple site for using iTunes to sync a video to an iPad: