Addition to existing website (Classifieds Section)

We are needing an addition of a classifieds section to our existing website which will be stand-alone. Also it will need to have a search engine built in that constantly adds listings from other classified websites. EG: We want to have a classified website that adds listings from other websites that we will specify to winning bidder. The classifieds website needs to have the same functionality and similar look with space for banners and google advertising. Ideally someone has already got an existing classifieds website that is very similar and it can be adapted to suit us. There will also be a lot of on-going work for the winning bidder to constantly add and improve. Only serious bidders should apply as we want someone that can do a great job and work with us long term