Article site selling content with author login capabilities at my site

Hi, I'm looking for a programmer who can add a web content section to my site that offers articles for sale with a PayPal shopping cart. Also, I have designed the entire site in HTML already, so I'm not sure if a PHP section can be added without changing the other sections of my site? If so I will need to start with a new domain name. The site will need to operate similar to [login to view URL] - their article and buyer sections except it will not be on a large scale as their site is. I will work with maybe 10 or 15 writers closely instead of allowing thousands to join. I want it to be automated so authors can submit their articles and the pricing will be automatically integrated with a PayPal shopping cart for people to buy the content. Authors should be able to submit their articles in text format on the form as well. When buyers purchase the articles, the "full rights" articles should disappear from the site immediately, and the articles should be available for immediate download in text format on any purchase. Also, buyers should be able to buy content with full rights or usage, whichever they prefer. I hope this gives a good idea of what I am looking for. Thanks! Candice