Add CMS functionality to website

I manage a website for a housing association. To the registered owners of the housing association I need to provide the ability to post ads on the website for personal items that they wish to sell (including their own homes) or acquire and I want them also to be able to post announcements for viewing by the other owners. I need the postings to be moderated. To save cost I am suggesting that the successful bidder find a suitable free or close to free script and then adapt it to and install it on the website