Out of the gate, I'd recommend performing a simple cost benefit analysis.

1) Figure out what roles you would need to hire (FTE/part time pay roll)
2) Understand the lifecycle of these roles - is it ongoing work? Set up work and out the door?
3) How bought in is our organization on SEM/SEO?
4) What life stage is our a) company, and b) marketing, at?

Ask yourself those Q's.

A few final follow on comments.

> Once you calculate your costs from a FTE/part timer standpoint, you'll be able to take that and compare it against external consultants/agencies.
> If you are an early stage company, hire a single part-time consultant early. Avoid taking on FTE/expensive agency overhead.
> Think about how much of a specialist you need. Do you need technical (e.g. front-end programming) capabilities? Or is it more of a generic "let's throw Yoast SEO plugin up, write/promote a ton of content and call it a day" type need?
> Question external consultants and agencies mercilessly. Have them do live demo's to walk through their proprietary algorithms, tools, etc.
> Firing external consultants/agencies is way easier than parting with FTE head counts (see point #2 again)

Best of luck!