You can compare iPad models here:
iPad - Compare Models - Apple
If downloading videos, space will be important. As internal storage is fixed for the life of the iPad - and cannot be expanded or updated - you should buy an iPad with as much storage as your budget will allow. It is far better to have more storage than you need, than to need more storage than you have.
Within the Apple Support Communities we see regular tales of woe and disappointment when storage requirements are exceeded, necessitating replacement of the device; by contrast, we never see complaint of having too much available storage. It is relatively easy to fill limited internal storage space; perhaps consider higher tiers of storage as providing room for growth as your needs develop or mature - potentially extending the useful usable life of the device.
Consider also the functional differences between WiFi Only and WiFi+Cellular models of iPad. Only WiFi+Cellular models include GPS capabilities - which may impact any Apps or features that require accurate and consistent location data, such as Mapping. Purchase of a WiFi+Cellular model does not commit you to using Cellular connections, but provides this flexibility should it be needed. WiFi Only models of iPad have no Cellular or GPS functions at all.
