You need to think more about the process of how to do it. Using something like PayPal or Google Checkout would be an easy solution.
Home Page > Select Movie > Preview? > Payment Gateway > Return to Movie download (make this page require a session cookie that is set upon payment so that they can't just hotlink back)
Some other things to consider:
What if I buy a movie but the download fails and I have left the website? I need an account to log back into and I need a set amount of time to allow me to initiate the download again, otherwise I'm an unhappy customer.
Bandwidth & hosting costs for the website, movies are big!
I would use PHP & MySQL but that's just because I'm comfortable with them.
Just letting you know, this is going to be a massive involved website. Be prepared. And quote for it!