Download size/time of iTunes Movies and Rentals + Apple TV use What is the range of file sizes that are downloaded when purchasing or renting a movie via iTunes? This would be useful in estimating the time to download.

Apple's help is very vague just saying that "The download time depends on the speed of your Internet connection." without providing any references to file size or anything to help work out a time.

Do rentals buffer and allow immediate playback - or is the complete file required?

With the new Apple TV not having local storage - how does it deal with movie purchases or rentals? Does it require a computer with home sharing enabled to download it to?
A single movie will usually be between 1-2G. The amount it will want to buffer will depend on the speed of the buffering.

Having a reliable sustained download speed of about 300K/s then it should buffer and start playing within a few seconds.