As John mentioned, there are two ways to think about SEO. The first is a one-time investment during a site redesign, to make sure your site is structured based on SEO best practices. But this only gets you a small part of the way there if you really want your content to rank well.
SEO should be a factor in every piece of content that goes up on your site, like blog posts or landing pages. This means mainly three things:
- Use language in your copy that's consistent with the keywords you're trying to rank for
- Include lots of internal linking between sections and pages on your site
- Consider all of the places you could get inbound links to your content from other sites
SEO is surprisingly simple, but it must be considered whenever you add any new content to your site in order for you to get the most value from it.
I'm not sure that having an outside SEO consultant would really help much with these day-to-day SEO considerations.
By the way, if you decide you'd like to have some outside help, HubSpot put together a great guide on how to spot a bad SEO consultant that you might find useful: