If i purchase or rent a movie and downloaded it into my iPad, do i need a wifi to watch it?

You definitely do not need WiFi to watch a purchased movie once it has been downloaded to your iPad. I have several movies that I purchased for watching on flights and every one of them worked fine without WiFi.

You should also be able to rent a movie and watch it without WiFi as long as the movie has fully downloaded properly. I have read about users having mixed results with this and that is why I said that you should be able to do it.