If you buy the original DVD you can convert it using Handbrake into an x264 video in a m4v container with dolby digital 5.1 audio.
Only Apple hardware like the ATV support this m4v x264/ac3 5.1 combo.
Handbrake comes with an AppleTV profile and just use the constant quality mode of 70% for very good image quality.
iTunes will accept the files and the ATV will play them.
Also you'd run into the problem of the ATV having very limited media support, I dont even know of any online stores that sell drm-free films and the videos would have to be compatible with iTunes in the first place.