employment posting site web application

I have a customer who wants me to create a site in which a very specific field of potential employers can pay to post job openings. He will then act as a talent scout of sorts, working of potential employees and matching them appropriately. The money will exclusively come from employers posting their job openings (with maybe adding a blanket yearly registration fee later). The site will be set up in such a way that all the specific areas of the field will be entered by my customer, under which the employers can post their jobs. There needs to be a 30 day and 90 day option to post a job opening. And there needs to be a clean, hassle-free way to take money, including recurring payments.

I am hoping to find some sort of pre-existing system that I can take and modify rather than build this thing from the scratch. Are there any suggestions as to what system could be tweaked to do all the things I need? Thanks.