Classified Ad website creation, wordpress, php, database, Backend Critical

We are looking for someone to create a great useable classified website. One of our business partners is a strong graphic design artist however we are looking for the functionaility (backend and design both). A comparable website would be www.barterquest.com. On the site we are looking for the ability to search via zip (geo) in short this will be a global site however we want customers to choose how far they want to search very easily, they need to have categories of items, have a live streaming comment section on each item (incorporate social media for each item...live comments is critical so customers can publicly inquire about details of the item for sale or trade) All users can join for $1 per month and that will allow them to post unlimited ads. We do want to offer some premium services such as featured ads or HD images etc. Viewers can browse for free however to see item contact info or certain details they too must pay that same subscriber $1. Because of the features we believe it may be difficult and costly to design from scratch so our thought was maybe wordpress with plugin's. But we are open to suggestions. The site must be clean, modern, not buggy and be able to handle 1000's of visitors daily. We intended on HEAVILY marketing the site so it must end up professional. We are also open to purchasing a backend product and having the front end heavily customized such as [login to view URL]