Build a Classified Mobile Application

We are looking to have a classified application built that will take the classified website that we have and translate everything over to the mobile application. Speed is extremely important in this case and it is also very important that the design is done extremely well. We would like to have your suggestions regarding design and development. This application must also run natively on iPhone as well as Android. Should be extremely fast and you must also make sure we use push notification system as well.