classified listing site

I want to build a classifieds site, it should allow me to list unlimited sub categories under categories, for different regions and again sub regions under different regions. The site should be very flexible and should be easily extendable. It should be modular that is extending the site should mean only adding additional modules to extend the capability and previous code should be reusable.. Administration area should be able to provide for dynmic tables..(adding additional fields from admin area) For example a table storing listing for a mobile product may require additinal column like coments form the owner about the condition of mobile(this is just an example to illustrate my point on extending the table, in actuality we will have this column) in the future.. so the admin area should provie for adding that kind of database flexibility. The data and content of the site should be available to other sites( as a web service or rss feeds etc.) There are already scripts avaiable for various functionalities of this site, there is also a classifieds starter kit available from microsoft and the dotnetnuke cms. So this may be basically an integeration job or if u have a valid reason for doing this from scratch, ur suggestions are welcome. I am looking for an experienced web developer who is well versed with db design, performance optimization , and xml , web services etc. Please provide details about how u would go about this job(project proposal- what would u exactly provide)... additional terms and condition can be discussed afer intial contact if your proposal suits. This is not a time consuming job for an experienced web developer, so bid reasonably.