since the install file for iOS is under 100MB I don't think the program Kodi itself will be a factor when deciding how much GB you want to buy.
However your local media collection (music and videos) might very well be a factor, but do you want to store all your files on the iPad? or are you using a NAS or similar to share your media?
Generally speaking: the more info you give us what you want and what your question is, the more specific and helpful the answer given will be (theoretically, at least )