Build a classified ad site

I want to build a classified ad site, and be able to have ads placed on the site. I was looking at using [login to view URL] as the open source code, but wanted to create a customized ad option to be able to place ads on the site. similarly to: [login to view URL] (as u can see there are banner ads on top, on the side as well as on the bottom, and in between other classified ads.) The only thing I want to integrate is ads. I want to be able to offer ads at the top of the home page, in between other classified ad spots, and on the right hand side of the site. I also want to be able to place ads on the backend of the user. When a user logs into their account I want to have ads placed in there as well. I would also like ads on the login page. Those are the locations I want to see ads on. As for control, I would like to be able to have the option to either put all the same ads on all the sections, or be able to control each section individually. For instance, if automotive section has no purchased ads, I would like it to have the same ads that are currently running on the site. If automotive has ads sold, and dating has ads sold, but real estate doesnt have any, then I would like to be able to place either auto ads, dating ads or sell ad space ads. Basically, I would like to be able to have control over what ad goes where, and have the ability to automatically place blank ads spaces when ads are not sold. i know it says the budget is between 250-750, but really I would not want to spend more than 300-400$ MAX. thank you. please only serious replies, so to know that you read the entire post please start your message with "Hello Sahar,"