I'd recommend asking yourself and/or internal team two basic, but critical questions:

1. How up to date is the internal team on current SEO best practices and allowed tactics?

2. Does the internal team at all question their ability to execute successfully on SEO?

If the team is questionable on either answer, an outside SEO consultant should be considered. I've seen many websites get hurt by well intentioned internal marketers.

If these internal staff is up to date on SEO best practices, feels confident in their abilities, and has time to devote to the project, then they may be more then capable of executing successfully.