What do you mean when you talk about SEO? It's really simplified thing in nowadays.

There is a pretty consize list of things which really matter, it's:

1) Content;
2) SXO (Search User Experience + Site User Experience);
3) On Page Optimization.

1 and 2 point give you 95% of effect, 3 - not more than 5%.

First point (content) should be done in house. You can outsource some writing, but ideas and plan is a part of your strategy.

Second point (SXO) - it's again about you and your product. You can hire some guys to improve or check it, but there is no need it in constant works.

Third point (On Page Optimization) - in nowadays it's really simplified and I don't think that there is a job for outsource team.

Write texts for people, not for machine. But as only you know your customer, it's obvious that you should make this job.

As an conclusion:

80% - In House;
20% - It's mostly about checking, improving and some writing.

One more advice. Don't believe old-fashioned SEO agencies and freelancers. In most cases they "help" to make your site penalized, not boost it.

Have a nice day.