If your site is focused on providing great value for a particular audience, then your website should have great content and a good website structure that is user friendly. If this is the case, then search engines will find your website and include your information in their search results.
I would spend some money on an SEO consultant to make sure the structure of your website is optimized for search engines.
A PPC campaign doesn't make sense if you don't have specific keyword to target, unless using a display campaign.
I have many different sites that don't focus on one particular keyword, these sites receive thousands of search engine referals from all sorts of search terms.
My sites include directories, news publications, product catalogs and informational websites. For all these websites Organic search is the number 1 referrer of traffic.
So, yes SEO is a good choice, but you may not need a continuos campaign, just someone to get the structure correct in the beginning.