Haven't tested it myself yet but AdultCentro.com seems to have a good setup for creating your own site but not sure if it has pay-per-download model - might be worth giving them a call and explain the rev model your are interested in. They seem to have daily download limitations and have templates for dvd/streaming store so it may well be possible to do pay-per-stream.
http://publisher.adultcentro.com/tour/billing
I think your best bet maybe would allow users to 'rent' full videos in a streaming format - a lot easier to protect from distribution and can keep them coming back later for the same vid but have to pay again ;)
A quick search for sites like these use this model:
http://theater.aebn.net/dispatcher/m...heaterId=44205
Anyway, hope that helps a bit!